diff --git a/spec/unit/models/event.spec.ts b/spec/unit/models/event.spec.ts
index f32256253a..65bd26ece2 100644
--- a/spec/unit/models/event.spec.ts
+++ b/spec/unit/models/event.spec.ts
@@ -20,6 +20,7 @@ import { type IContent, MatrixEvent, MatrixEventEvent } from "../../../src/model
 import { emitPromise } from "../../test-utils/test-utils";
 import {
     type IAnnotatedPushRule,
+    type IStickyEvent,
     type MatrixClient,
     PushRuleActionName,
     Room,
@@ -598,6 +599,39 @@ describe("MatrixEvent", () => {
         expect(stateEvent.isState()).toBeTruthy();
         expect(stateEvent.threadRootId).toBeUndefined();
     });
+
+    it("should calculate sticky duration correctly", async () => {
+        const evData: IStickyEvent = {
+            event_id: "$event_id",
+            type: "some_state_event",
+            content: {},
+            sender: "@alice:example.org",
+            origin_server_ts: 50,
+            msc4354_sticky: {
+                duration_ms: 1000,
+            },
+            unsigned: {
+                msc4354_sticky_duration_ttl_ms: 5000,
+            },
+        };
+        try {
+            jest.useFakeTimers();
+            jest.setSystemTime(50);
+            // Prefer unsigned
+            expect(new MatrixEvent({ ...evData } satisfies IStickyEvent).unstableStickyExpiresAt).toEqual(5050);
+            // Fall back to `duration_ms`
+            expect(
+                new MatrixEvent({ ...evData, unsigned: undefined } satisfies IStickyEvent).unstableStickyExpiresAt,
+            ).toEqual(1050);
+            // Prefer current time if `origin_server_ts` is more recent.
+            expect(
+                new MatrixEvent({ ...evData, unsigned: undefined, origin_server_ts: 5000 } satisfies IStickyEvent)
+                    .unstableStickyExpiresAt,
+            ).toEqual(1050);
+        } finally {
+            jest.useRealTimers();
+        }
+    });
 });
 
 function mainTimelineLiveEventIds(room: Room): Array<string> {
diff --git a/spec/unit/models/room-sticky-events.spec.ts b/spec/unit/models/room-sticky-events.spec.ts
new file mode 100644
index 0000000000..a51fe461c2
--- /dev/null
+++ b/spec/unit/models/room-sticky-events.spec.ts
@@ -0,0 +1,262 @@
+import { type IStickyEvent, MatrixEvent } from "../../../src";
+import { RoomStickyEventsStore, RoomStickyEventsEvent } from "../../../src/models/room-sticky-events";
+
+describe("RoomStickyEvents", () => {
+    let stickyEvents: RoomStickyEventsStore;
+    const emitSpy: jest.Mock = jest.fn();
+    const stickyEvent: IStickyEvent = {
+        event_id: "$foo:bar",
+        room_id: "!roomId",
+        type: "org.example.any_type",
+        msc4354_sticky: {
+            duration_ms: 15000,
+        },
+        content: {
+            msc4354_sticky_key: "foobar",
+        },
+        sender: "@alice:example.org",
+        origin_server_ts: Date.now(),
+        unsigned: {},
+    };
+
+    beforeEach(() => {
+        emitSpy.mockReset();
+        stickyEvents = new RoomStickyEventsStore();
+        stickyEvents.on(RoomStickyEventsEvent.Update, emitSpy);
+    });
+
+    afterEach(() => {
+        stickyEvents?.clear();
+    });
+
+    describe("addStickyEvents", () => {
+        it("should allow adding an event without a msc4354_sticky_key", () => {
+            stickyEvents.addStickyEvents([new MatrixEvent({ ...stickyEvent, content: {} })]);
+            expect([...stickyEvents.getStickyEvents()]).toHaveLength(1);
+        });
+        it("should not allow adding an event without a msc4354_sticky property", () => {
+            stickyEvents.addStickyEvents([new MatrixEvent({ ...stickyEvent, msc4354_sticky: undefined })]);
+            expect([...stickyEvents.getStickyEvents()]).toHaveLength(0);
+            stickyEvents.addStickyEvents([
+                new MatrixEvent({ ...stickyEvent, msc4354_sticky: { duration_ms: undefined } as any }),
+            ]);
+            expect([...stickyEvents.getStickyEvents()]).toHaveLength(0);
+        });
+        it("should not allow adding an event without a sender", () => {
+            stickyEvents.addStickyEvents([new MatrixEvent({ ...stickyEvent, sender: undefined })]);
+            expect([...stickyEvents.getStickyEvents()]).toHaveLength(0);
+        });
+        it("should not allow adding an event with an invalid sender", () => {
+            stickyEvents.addStickyEvents([new MatrixEvent({ ...stickyEvent, sender: "not_a_real_sender" })]);
+            expect([...stickyEvents.getStickyEvents()]).toHaveLength(0);
+        });
+        it("should ignore old events", () => {
+            stickyEvents.addStickyEvents([
+                new MatrixEvent({ ...stickyEvent, origin_server_ts: 0, msc4354_sticky: { duration_ms: 1 } }),
+            ]);
+            expect([...stickyEvents.getStickyEvents()]).toHaveLength(0);
+        });
+        it("should be able to just add an event", () => {
+            const originalEv = new MatrixEvent({ ...stickyEvent });
+            stickyEvents.addStickyEvents([originalEv]);
+            expect([...stickyEvents.getStickyEvents()]).toEqual([originalEv]);
+        });
+        it("should not replace events on ID tie break", () => {
+            const originalEv = new MatrixEvent({ ...stickyEvent });
+            stickyEvents.addStickyEvents([originalEv]);
+            stickyEvents.addStickyEvents([
+                new MatrixEvent({
+                    ...stickyEvent,
+                    event_id: "$abc:bar",
+                }),
+            ]);
+            expect([...stickyEvents.getStickyEvents()]).toEqual([originalEv]);
+        });
+        it("should not replace a newer event with an older event", () => {
+            const originalEv = new MatrixEvent({ ...stickyEvent });
+            stickyEvents.addStickyEvents([originalEv]);
+            stickyEvents.addStickyEvents([
+                new MatrixEvent({
+                    ...stickyEvent,
+                    origin_server_ts: 1,
+                }),
+            ]);
+            expect([...stickyEvents.getStickyEvents()]).toEqual([originalEv]);
+        });
+        it("should replace an older event with a newer event", () => {
+            const originalEv = new MatrixEvent({ ...stickyEvent, event_id: "$old" });
+            const newerEv = new MatrixEvent({
+                ...stickyEvent,
+                event_id: "$new",
+                origin_server_ts: Date.now() + 2000,
+            });
+            stickyEvents.addStickyEvents([originalEv]);
+            stickyEvents.addStickyEvents([newerEv]);
+            expect([...stickyEvents.getStickyEvents()]).toEqual([newerEv]);
+            expect(emitSpy).toHaveBeenCalledWith([], [{ current: newerEv, previous: originalEv }], []);
+        });
+        it("should allow multiple events with the same sticky key for different event types", () => {
+            const originalEv = new MatrixEvent({ ...stickyEvent });
+            const anotherEv = new MatrixEvent({
+                ...stickyEvent,
+                type: "org.example.another_type",
+            });
+            stickyEvents.addStickyEvents([originalEv, anotherEv]);
+            expect([...stickyEvents.getStickyEvents()]).toEqual([originalEv, anotherEv]);
+        });
+
+        it("should emit when a new sticky event is added", () => {
+            stickyEvents.on(RoomStickyEventsEvent.Update, emitSpy);
+            const ev = new MatrixEvent({
+                ...stickyEvent,
+            });
+            stickyEvents.addStickyEvents([ev]);
+            expect([...stickyEvents.getStickyEvents()]).toEqual([ev]);
+            expect(emitSpy).toHaveBeenCalledWith([ev], [], []);
+        });
+        it("should emit when a new unkeyed sticky event is added", () => {
+            stickyEvents.on(RoomStickyEventsEvent.Update, emitSpy);
+            const ev = new MatrixEvent({
+                ...stickyEvent,
+                content: {},
+            });
+            stickyEvents.addStickyEvents([ev]);
+            expect([...stickyEvents.getStickyEvents()]).toEqual([ev]);
+            expect(emitSpy).toHaveBeenCalledWith([ev], [], []);
+        });
+    });
+
+    describe("getStickyEvents", () => {
+        it("should have zero sticky events", () => {
+            expect([...stickyEvents.getStickyEvents()]).toHaveLength(0);
+        });
+        it("should contain a sticky event", () => {
+            const ev = new MatrixEvent({
+                ...stickyEvent,
+            });
+            stickyEvents.addStickyEvents([ev]);
+            expect([...stickyEvents.getStickyEvents()]).toEqual([ev]);
+        });
+        it("should contain two sticky events", () => {
+            const ev = new MatrixEvent({
+                ...stickyEvent,
+            });
+            const ev2 = new MatrixEvent({
+                ...stickyEvent,
+                sender: "@fibble:bobble",
+                content: {
+                    msc4354_sticky_key: "bibble",
+                },
+            });
+            stickyEvents.addStickyEvents([ev, ev2]);
+            expect([...stickyEvents.getStickyEvents()]).toEqual([ev, ev2]);
+        });
+    });
+
+    describe("getKeyedStickyEvent", () => {
+        it("should have zero sticky events", () => {
+            expect(
+                stickyEvents.getKeyedStickyEvent(
+                    stickyEvent.sender,
+                    stickyEvent.type,
+                    stickyEvent.content.msc4354_sticky_key!,
+                ),
+            ).toBeUndefined();
+        });
+        it("should return a sticky event", () => {
+            const ev = new MatrixEvent({
+                ...stickyEvent,
+            });
+            stickyEvents.addStickyEvents([ev]);
+            expect(
+                stickyEvents.getKeyedStickyEvent(
+                    stickyEvent.sender,
+                    stickyEvent.type,
+                    stickyEvent.content.msc4354_sticky_key!,
+                ),
+            ).toEqual(ev);
+        });
+    });
+
+    describe("getUnkeyedStickyEvent", () => {
+        it("should have zero sticky events", () => {
+            expect(stickyEvents.getUnkeyedStickyEvent(stickyEvent.sender, stickyEvent.type)).toEqual([]);
+        });
+        it("should return a sticky event", () => {
+            const ev = new MatrixEvent({
+                ...stickyEvent,
+                content: {
+                    msc4354_sticky_key: undefined,
+                },
+            });
+            stickyEvents.addStickyEvents([ev]);
+            expect(stickyEvents.getUnkeyedStickyEvent(stickyEvent.sender, stickyEvent.type)).toEqual([ev]);
+        });
+    });
+
+    describe("cleanExpiredStickyEvents", () => {
+        beforeAll(() => {
+            jest.useFakeTimers();
+        });
+        afterAll(() => {
+            jest.useRealTimers();
+        });
+
+        it("should emit when a sticky event expires", () => {
+            jest.setSystemTime(1000);
+            const ev = new MatrixEvent({
+                ...stickyEvent,
+                origin_server_ts: 0,
+            });
+            const evLater = new MatrixEvent({
+                ...stickyEvent,
+                event_id: "$baz:bar",
+                sender: "@bob:example.org",
+                origin_server_ts: 1000,
+            });
+            stickyEvents.addStickyEvents([ev, evLater]);
+            const emitSpy = jest.fn();
+            stickyEvents.on(RoomStickyEventsEvent.Update, emitSpy);
+            jest.advanceTimersByTime(15000);
+            expect(emitSpy).toHaveBeenCalledWith([], [], [ev]);
+            // Then expire the next event
+            jest.advanceTimersByTime(1000);
+            expect(emitSpy).toHaveBeenCalledWith([], [], [evLater]);
+        });
+        it("should emit two events when both expire at the same time", () => {
+            const emitSpy = jest.fn();
+            stickyEvents.on(RoomStickyEventsEvent.Update, emitSpy);
+            jest.setSystemTime(0);
+            const ev1 = new MatrixEvent({
+                ...stickyEvent,
+                event_id: "$eventA",
+                origin_server_ts: 0,
+            });
+            const ev2 = new MatrixEvent({
+                ...stickyEvent,
+                event_id: "$eventB",
+                content: {
+                    msc4354_sticky_key: "key_2",
+                },
+                origin_server_ts: 0,
+            });
+            stickyEvents.addStickyEvents([ev1, ev2]);
+            expect(emitSpy).toHaveBeenCalledWith([ev1, ev2], [], []);
+            jest.advanceTimersByTime(15000);
+            expect(emitSpy).toHaveBeenCalledWith([], [], [ev1, ev2]);
+        });
+        it("should emit when a unkeyed sticky event expires", () => {
+            const emitSpy = jest.fn();
+            stickyEvents.on(RoomStickyEventsEvent.Update, emitSpy);
+            jest.setSystemTime(0);
+            const ev = new MatrixEvent({
+                ...stickyEvent,
+                content: {},
+                origin_server_ts: Date.now(),
+            });
+            stickyEvents.addStickyEvents([ev]);
+            jest.advanceTimersByTime(15000);
+            expect(emitSpy).toHaveBeenCalledWith([], [], [ev]);
+        });
+    });
+});
diff --git a/spec/unit/sync-accumulator.spec.ts b/spec/unit/sync-accumulator.spec.ts
index 22d3336007..d9333b4346 100644
--- a/spec/unit/sync-accumulator.spec.ts
+++ b/spec/unit/sync-accumulator.spec.ts
@@ -26,6 +26,7 @@ import {
     type ILeftRoom,
     type IRoomEvent,
     type IStateEvent,
+    type IStickyEvent,
     type IStrippedState,
     type ISyncResponse,
     SyncAccumulator,
@@ -1067,6 +1068,67 @@ describe("SyncAccumulator", function () {
             );
         });
     });
+
+    describe("MSC4354 sticky events", () => {
+        function stickyEvent(ts = 0): IStickyEvent {
+            const msgData = msg("test", "test text");
+            return {
+                ...msgData,
+                msc4354_sticky: {
+                    duration_ms: 1000,
+                },
+                origin_server_ts: ts,
+            };
+        }
+
+        beforeAll(() => {
+            jest.useFakeTimers();
+        });
+
+        afterAll(() => {
+            jest.useRealTimers();
+        });
+
+        it("should accumulate sticky events", () => {
+            jest.setSystemTime(0);
+            const ev = stickyEvent();
+            sa.accumulate(
+                syncSkeleton({
+                    msc4354_sticky: {
+                        events: [ev],
+                    },
+                }),
+            );
+            expect(sa.getJSON().roomsData[Category.Join]["!foo:bar"].msc4354_sticky?.events).toEqual([ev]);
+        });
+        it("should clear stale sticky events", () => {
+            jest.setSystemTime(1000);
+            const ev = stickyEvent(1000);
+            sa.accumulate(
+                syncSkeleton({
+                    msc4354_sticky: {
+                        events: [ev],
+                    },
+                }),
+            );
+            expect(sa.getJSON().roomsData[Category.Join]["!foo:bar"].msc4354_sticky?.events).toEqual([ev]);
+            jest.setSystemTime(2000); // Expire the event
+            sa.accumulate(syncSkeleton({}));
+            expect(sa.getJSON().roomsData[Category.Join]["!foo:bar"].msc4354_sticky?.events).toBeUndefined();
+        });
+
+        it("clears stale sticky events that pretend to be from the distant future", () => {
+            jest.setSystemTime(0);
+            const eventFarInTheFuture = stickyEvent(999999999999);
+            sa.accumulate(syncSkeleton({ msc4354_sticky: { events: [eventFarInTheFuture] } }));
+            expect(sa.getJSON().roomsData[Category.Join]["!foo:bar"].msc4354_sticky?.events).toEqual([
+                eventFarInTheFuture,
+            ]);
+            jest.setSystemTime(1000); // Expire the event
+            sa.accumulate(syncSkeleton({}));
+            expect(sa.getJSON().roomsData[Category.Join]["!foo:bar"].msc4354_sticky?.events).toBeUndefined();
+        });
+    });
 });
 
 function syncSkeleton(
diff --git a/src/@types/requests.ts b/src/@types/requests.ts
index b985bec293..b6fd916cd2 100644
--- a/src/@types/requests.ts
+++ b/src/@types/requests.ts
@@ -94,19 +94,20 @@ export interface ISendEventResponse {
     event_id: string;
 }
 
-export type TimeoutDelay = {
-    delay: number;
-};
-
-export type ParentDelayId = {
-    parent_delay_id: string;
-};
-
-export type SendTimeoutDelayedEventRequestOpts = TimeoutDelay & Partial<ParentDelayId>;
-export type SendActionDelayedEventRequestOpts = ParentDelayId;
-
-export type SendDelayedEventRequestOpts = SendTimeoutDelayedEventRequestOpts | SendActionDelayedEventRequestOpts;
-
+export type SendDelayedEventRequestOpts = { parent_delay_id: string } | { delay: number; parent_delay_id?: string };
+
+export function isSendDelayedEventRequestOpts(opts: object): opts is SendDelayedEventRequestOpts {
+    if ("parent_delay_id" in opts && typeof opts.parent_delay_id !== "string") {
+        // Invalid type, reject
+        return false;
+    }
+    if ("delay" in opts && typeof opts.delay !== "number") {
+        // Invalid type, reject.
+        return true;
+    }
+    // At least one of these fields must be specified.
+    return "delay" in opts || "parent_delay_id" in opts;
+}
 export type SendDelayedEventResponse = {
     delay_id: string;
 };
diff --git a/src/client.ts b/src/client.ts
index 52029ce475..580f2edef6 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -105,6 +105,7 @@ import {
 import { RoomMemberEvent, type RoomMemberEventHandlerMap } from "./models/room-member.ts";
 import { type IPowerLevelsContent, type RoomStateEvent, type RoomStateEventHandlerMap } from "./models/room-state.ts";
 import {
+    isSendDelayedEventRequestOpts,
     type DelayedEventInfo,
     type IAddThreePidOnlyBody,
     type IBindThreePidBody,
@@ -246,7 +247,7 @@ import {
     validateAuthMetadataAndKeys,
 } from "./oidc/index.ts";
 import { type EmptyObject } from "./@types/common.ts";
-import { UnsupportedDelayedEventsEndpointError } from "./errors.ts";
+import { UnsupportedDelayedEventsEndpointError, UnsupportedStickyEventsEndpointError } from "./errors.ts";
 
 export type Store = IStore;
 
@@ -551,6 +552,7 @@ export const UNSTABLE_MSC2666_MUTUAL_ROOMS = "uk.half-shot.msc2666.mutual_rooms"
 export const UNSTABLE_MSC2666_QUERY_MUTUAL_ROOMS = "uk.half-shot.msc2666.query_mutual_rooms";
 
 export const UNSTABLE_MSC4140_DELAYED_EVENTS = "org.matrix.msc4140";
+export const UNSTABLE_MSC4354_STICKY_EVENTS = "org.matrix.msc4354";
 
 export const UNSTABLE_MSC4133_EXTENDED_PROFILES = "uk.tcpip.msc4133";
 export const STABLE_MSC4133_EXTENDED_PROFILES = "uk.tcpip.msc4133.stable";
@@ -2681,7 +2683,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
         }
 
         this.addThreadRelationIfNeeded(content, threadId, roomId);
-        return this.sendCompleteEvent(roomId, threadId, { type: eventType, content }, txnId);
+        return this.sendCompleteEvent({ roomId, threadId, eventObject: { type: eventType, content }, txnId });
     }
 
     /**
@@ -2718,12 +2720,13 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
      * @returns Promise which resolves: to an empty object `{}`
      * @returns Rejects: with an error response.
      */
-    private sendCompleteEvent(
-        roomId: string,
-        threadId: string | null,
-        eventObject: Partial<IEvent>,
-        txnId?: string,
-    ): Promise<ISendEventResponse>;
+    private sendCompleteEvent(params: {
+        roomId: string;
+        threadId: string | null;
+        eventObject: Partial<IEvent>;
+        queryDict?: QueryDict;
+        txnId?: string;
+    }): Promise<ISendEventResponse>;
     /**
      * Sends a delayed event (MSC4140).
      * @param eventObject - An object with the partial structure of an event, to which event_id, user_id, room_id and origin_server_ts will be added.
@@ -2732,29 +2735,29 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
      * @returns Promise which resolves: to an empty object `{}`
      * @returns Rejects: with an error response.
      */
-    private sendCompleteEvent(
-        roomId: string,
-        threadId: string | null,
-        eventObject: Partial<IEvent>,
-        delayOpts: SendDelayedEventRequestOpts,
-        txnId?: string,
-    ): Promise<SendDelayedEventResponse>;
-    private sendCompleteEvent(
-        roomId: string,
-        threadId: string | null,
-        eventObject: Partial<IEvent>,
-        delayOptsOrTxnId?: SendDelayedEventRequestOpts | string,
-        txnIdOrVoid?: string,
-    ): Promise<ISendEventResponse | SendDelayedEventResponse> {
-        let delayOpts: SendDelayedEventRequestOpts | undefined;
-        let txnId: string | undefined;
-        if (typeof delayOptsOrTxnId === "string") {
-            txnId = delayOptsOrTxnId;
-        } else {
-            delayOpts = delayOptsOrTxnId;
-            txnId = txnIdOrVoid;
-        }
-
+    private sendCompleteEvent(params: {
+        roomId: string;
+        threadId: string | null;
+        eventObject: Partial<IEvent>;
+        delayOpts: SendDelayedEventRequestOpts;
+        queryDict?: QueryDict;
+        txnId?: string;
+    }): Promise<SendDelayedEventResponse>;
+    private sendCompleteEvent({
+        roomId,
+        threadId,
+        eventObject,
+        delayOpts,
+        queryDict,
+        txnId,
+    }: {
+        roomId: string;
+        threadId: string | null;
+        eventObject: Partial<IEvent>;
+        delayOpts?: SendDelayedEventRequestOpts;
+        queryDict?: QueryDict;
+        txnId?: string;
+    }): Promise<SendDelayedEventResponse | ISendEventResponse> {
         if (!txnId) {
             txnId = this.makeTxnId();
         }
@@ -2797,7 +2800,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
 
         const type = localEvent.getType();
         this.logger.debug(
-            `sendEvent of type ${type} in ${roomId} with txnId ${txnId}${delayOpts ? " (delayed event)" : ""}`,
+            `sendEvent of type ${type} in ${roomId} with txnId ${txnId}${delayOpts ? " (delayed event)" : ""}${queryDict ? " query params: " + JSON.stringify(queryDict) : ""}`,
         );
 
         localEvent.setTxnId(txnId);
@@ -2815,9 +2818,9 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
                 return Promise.reject(new Error("Event blocked by other events not yet sent"));
             }
 
-            return this.encryptAndSendEvent(room, localEvent);
+            return this.encryptAndSendEvent(room, localEvent, queryDict);
         } else {
-            return this.encryptAndSendEvent(room, localEvent, delayOpts);
+            return this.encryptAndSendEvent(room, localEvent, delayOpts, queryDict);
         }
     }
 
@@ -2825,7 +2828,11 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
      * encrypts the event if necessary; adds the event to the queue, or sends it; marks the event as sent/unsent
      * @returns returns a promise which resolves with the result of the send request
      */
-    protected async encryptAndSendEvent(room: Room | null, event: MatrixEvent): Promise<ISendEventResponse>;
+    protected async encryptAndSendEvent(
+        room: Room | null,
+        event: MatrixEvent,
+        queryDict?: QueryDict,
+    ): Promise<ISendEventResponse>;
     /**
      * Simply sends a delayed event without encrypting it.
      * TODO: Allow encrypted delayed events, and encrypt them properly
@@ -2836,16 +2843,20 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
         room: Room | null,
         event: MatrixEvent,
         delayOpts: SendDelayedEventRequestOpts,
-    ): Promise<SendDelayedEventResponse>;
+        queryDict?: QueryDict,
+    ): Promise<ISendEventResponse>;
     protected async encryptAndSendEvent(
         room: Room | null,
         event: MatrixEvent,
-        delayOpts?: SendDelayedEventRequestOpts,
+        delayOptsOrQuery?: SendDelayedEventRequestOpts | QueryDict,
+        queryDict?: QueryDict,
     ): Promise<ISendEventResponse | SendDelayedEventResponse> {
-        if (delayOpts) {
-            return this.sendEventHttpRequest(event, delayOpts);
+        let queryOpts = queryDict;
+        if (delayOptsOrQuery && isSendDelayedEventRequestOpts(delayOptsOrQuery)) {
+            return this.sendEventHttpRequest(event, delayOptsOrQuery, queryOpts);
+        } else if (!queryOpts) {
+            queryOpts = delayOptsOrQuery;
         }
-
         try {
             let cancelled: boolean;
             this.eventsBeingEncrypted.add(event.getId()!);
@@ -2881,7 +2892,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
             }
 
             if (!promise) {
-                promise = this.sendEventHttpRequest(event);
+                promise = this.sendEventHttpRequest(event, queryOpts);
                 if (room) {
                     promise = promise.then((res) => {
                         room.updatePendingEvent(event, EventStatus.SENT, res["event_id"]);
@@ -2996,14 +3007,16 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
         }
     }
 
-    private sendEventHttpRequest(event: MatrixEvent): Promise<ISendEventResponse>;
+    private sendEventHttpRequest(event: MatrixEvent, queryDict?: QueryDict): Promise<ISendEventResponse>;
     private sendEventHttpRequest(
         event: MatrixEvent,
         delayOpts: SendDelayedEventRequestOpts,
+        queryDict?: QueryDict,
     ): Promise<SendDelayedEventResponse>;
     private sendEventHttpRequest(
         event: MatrixEvent,
-        delayOpts?: SendDelayedEventRequestOpts,
+        queryOrDelayOpts?: SendDelayedEventRequestOpts | QueryDict,
+        queryDict?: QueryDict,
     ): Promise<ISendEventResponse | SendDelayedEventResponse> {
         let txnId = event.getTxnId();
         if (!txnId) {
@@ -3036,19 +3049,22 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
             path = utils.encodeUri("/rooms/$roomId/send/$eventType/$txnId", pathParams);
         }
 
+        const delayOpts =
+            queryOrDelayOpts && isSendDelayedEventRequestOpts(queryOrDelayOpts) ? queryOrDelayOpts : undefined;
+        const queryOpts = !delayOpts ? queryOrDelayOpts : queryDict;
         const content = event.getWireContent();
-        if (!delayOpts) {
-            return this.http.authedRequest<ISendEventResponse>(Method.Put, path, undefined, content).then((res) => {
-                this.logger.debug(`Event sent to ${event.getRoomId()} with event id ${res.event_id}`);
-                return res;
-            });
-        } else {
+        if (delayOpts) {
             return this.http.authedRequest<SendDelayedEventResponse>(
                 Method.Put,
                 path,
-                getUnstableDelayQueryOpts(delayOpts),
+                { ...getUnstableDelayQueryOpts(delayOpts), ...queryOpts },
                 content,
             );
+        } else {
+            return this.http.authedRequest<ISendEventResponse>(Method.Put, path, queryOpts, content).then((res) => {
+                this.logger.debug(`Event sent to ${event.getRoomId()} with event id ${res.event_id}`);
+                return res;
+            });
         }
     }
 
@@ -3105,16 +3121,16 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
             content[withRelTypesPropName] = opts.with_rel_types;
         }
 
-        return this.sendCompleteEvent(
+        return this.sendCompleteEvent({
             roomId,
             threadId,
-            {
+            eventObject: {
                 type: EventType.RoomRedaction,
                 content,
                 redacts: eventId,
             },
-            txnId as string,
-        );
+            txnId: txnId as string,
+        });
     }
 
     /**
@@ -3402,7 +3418,54 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
         }
 
         this.addThreadRelationIfNeeded(content, threadId, roomId);
-        return this.sendCompleteEvent(roomId, threadId, { type: eventType, content }, delayOpts, txnId);
+        return this.sendCompleteEvent({
+            roomId,
+            threadId,
+            eventObject: { type: eventType, content },
+            delayOpts,
+            txnId,
+        });
+    }
+
+    /**
+     * Send a delayed sticky timeline event.
+     *
+     * Note: This endpoint is unstable, and can throw an `Error`.
+     *   Check progress on [MSC4140](https://github.com/matrix-org/matrix-spec-proposals/pull/4140) and
+     *   [MSC4354](https://github.com/matrix-org/matrix-spec-proposals/pull/4354) for more details.
+     */
+    // eslint-disable-next-line
+    public async _unstable_sendStickyDelayedEvent<K extends keyof TimelineEvents>(
+        roomId: string,
+        stickDuration: number,
+        delayOpts: SendDelayedEventRequestOpts,
+        threadId: string | null,
+        eventType: K,
+        content: TimelineEvents[K] & { msc4354_sticky_key: string },
+        txnId?: string,
+    ): Promise<SendDelayedEventResponse> {
+        if (!(await this.doesServerSupportUnstableFeature(UNSTABLE_MSC4140_DELAYED_EVENTS))) {
+            throw new UnsupportedDelayedEventsEndpointError(
+                "Server does not support the delayed events API",
+                "getDelayedEvents",
+            );
+        }
+        if (!(await this.doesServerSupportUnstableFeature(UNSTABLE_MSC4354_STICKY_EVENTS))) {
+            throw new UnsupportedStickyEventsEndpointError(
+                "Server does not support the sticky events",
+                "sendStickyEvent",
+            );
+        }
+
+        this.addThreadRelationIfNeeded(content, threadId, roomId);
+        return this.sendCompleteEvent({
+            roomId,
+            threadId,
+            eventObject: { type: eventType, content },
+            queryDict: { "org.matrix.msc4354.sticky_duration_ms": stickDuration },
+            delayOpts,
+            txnId,
+        });
     }
 
     /**
@@ -3439,6 +3502,38 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
         return this.http.authedRequest(Method.Put, path, getUnstableDelayQueryOpts(delayOpts), content as Body, opts);
     }
 
+    /**
+     * Send a sticky timeline event.
+     *
+     * Note: This endpoint is unstable, and can throw an `Error`.
+     *   Check progress on [MSC4354](https://github.com/matrix-org/matrix-spec-proposals/pull/4354) for more details.
+     */
+    // eslint-disable-next-line
+    public async _unstable_sendStickyEvent<K extends keyof TimelineEvents>(
+        roomId: string,
+        stickDuration: number,
+        threadId: string | null,
+        eventType: K,
+        content: TimelineEvents[K] & { msc4354_sticky_key: string },
+        txnId?: string,
+    ): Promise<ISendEventResponse> {
+        if (!(await this.doesServerSupportUnstableFeature(UNSTABLE_MSC4354_STICKY_EVENTS))) {
+            throw new UnsupportedStickyEventsEndpointError(
+                "Server does not support the sticky events",
+                "sendStickyEvent",
+            );
+        }
+
+        this.addThreadRelationIfNeeded(content, threadId, roomId);
+        return this.sendCompleteEvent({
+            roomId,
+            threadId,
+            eventObject: { type: eventType, content },
+            queryDict: { "org.matrix.msc4354.sticky_duration_ms": stickDuration },
+            txnId,
+        });
+    }
+
     /**
      * Get all pending delayed events for the calling user.
      *
diff --git a/src/errors.ts b/src/errors.ts
index 8baf7979bc..672aee3bb4 100644
--- a/src/errors.ts
+++ b/src/errors.ts
@@ -54,7 +54,7 @@ export class ClientStoppedError extends Error {
 }
 
 /**
- * This error is thrown when the Homeserver does not support the delayed events enpdpoints.
+ * This error is thrown when the Homeserver does not support the delayed events endpoints.
  */
 export class UnsupportedDelayedEventsEndpointError extends Error {
     public constructor(
@@ -65,3 +65,16 @@ export class UnsupportedDelayedEventsEndpointError extends Error {
         this.name = "UnsupportedDelayedEventsEndpointError";
     }
 }
+
+/**
+ * This error is thrown when the Homeserver does not support the sticky events endpoints.
+ */
+export class UnsupportedStickyEventsEndpointError extends Error {
+    public constructor(
+        message: string,
+        public clientEndpoint: "sendStickyEvent" | "sendStickyStateEvent",
+    ) {
+        super(message);
+        this.name = "UnsupportedStickyEventsEndpointError";
+    }
+}
diff --git a/src/models/event.ts b/src/models/event.ts
index dba134b894..bc175ef0df 100644
--- a/src/models/event.ts
+++ b/src/models/event.ts
@@ -75,6 +75,7 @@ export interface IUnsigned {
     "transaction_id"?: string;
     "invite_room_state"?: StrippedState[];
     "m.relations"?: Record<RelationType | string, any>; // No common pattern for aggregated relations
+    "msc4354_sticky_duration_ttl_ms"?: number;
     [UNSIGNED_THREAD_ID_FIELD.name]?: string;
 }
 
@@ -96,6 +97,7 @@ export interface IEvent {
     membership?: Membership;
     unsigned: IUnsigned;
     redacts?: string;
+    msc4354_sticky?: { duration_ms: number };
 }
 
 export interface IAggregatedRelation {
@@ -213,6 +215,7 @@ export interface IMessageVisibilityHidden {
 }
 // A singleton implementing `IMessageVisibilityVisible`.
 const MESSAGE_VISIBLE: IMessageVisibilityVisible = Object.freeze({ visible: true });
+export const MAX_STICKY_DURATION_MS = 3600000;
 
 export enum MatrixEventEvent {
     /**
@@ -408,6 +411,17 @@ export class MatrixEvent extends TypedEventEmitter<MatrixEventEmittedEvents, Mat
 
     private readonly reEmitter: TypedReEmitter<MatrixEventEmittedEvents, MatrixEventHandlerMap>;
 
+    /**
+     * The timestamp for when this event should expire, in milliseconds.
+     * Prefers using the server-provided value, but will fall back to local calculation.
+     *
+     * This value is **safe** to use, as malicious start time and duration are appropriately capped.
+     *
+     * If the event is not a sticky event (or not supported by the server),
+     * then this returns `undefined`.
+     */
+    public readonly unstableStickyExpiresAt: number | undefined;
+
     /**
      * Construct a Matrix Event object
      *
@@ -447,8 +461,17 @@ export class MatrixEvent extends TypedEventEmitter<MatrixEventEmittedEvents, Mat
         // The fallback in these cases will be to use the origin_server_ts.
         // For EDUs, the origin_server_ts also is not defined so we use Date.now().
         const age = this.getAge();
-        this.localTimestamp = age !== undefined ? Date.now() - age : (this.getTs() ?? Date.now());
+        const now = Date.now();
+        this.localTimestamp = age !== undefined ? now - age : (this.getTs() ?? now);
         this.reEmitter = new TypedReEmitter(this);
+        if (this.unstableStickyInfo) {
+            if (this.unstableStickyInfo.duration_ttl_ms) {
+                this.unstableStickyExpiresAt = now + this.unstableStickyInfo.duration_ttl_ms;
+            } else {
+                // Bound the timestamp so it doesn't come from the future.
+                this.unstableStickyExpiresAt = Math.min(now, this.getTs()) + this.unstableStickyInfo.duration_ms;
+            }
+        }
     }
 
     /**
@@ -1739,6 +1762,24 @@ export class MatrixEvent extends TypedEventEmitter<MatrixEventEmittedEvents, Mat
     public setThreadId(threadId?: string): void {
         this.threadId = threadId;
     }
+
+    /**
+     * Unstable getter to try and get the sticky information for the event.
+     * If the event is not a sticky event (or not supported by the server),
+     * then this returns `undefined`.
+     *
+     * `duration_ms` is safely bounded to a hour.
+     */
+    public get unstableStickyInfo(): { duration_ms: number; duration_ttl_ms?: number } | undefined {
+        if (!this.event.msc4354_sticky?.duration_ms) {
+            return undefined;
+        }
+        return {
+            duration_ms: Math.min(MAX_STICKY_DURATION_MS, this.event.msc4354_sticky.duration_ms),
+            // This is assumed to be bounded server-side.
+            duration_ttl_ms: this.event.unsigned?.msc4354_sticky_duration_ttl_ms,
+        };
+    }
 }
 
 /* REDACT_KEEP_KEYS gives the keys we keep when an event is redacted
diff --git a/src/models/room-sticky-events.ts b/src/models/room-sticky-events.ts
new file mode 100644
index 0000000000..3e9e26a2bd
--- /dev/null
+++ b/src/models/room-sticky-events.ts
@@ -0,0 +1,241 @@
+import { logger as loggerInstance } from "../logger.ts";
+import { type MatrixEvent } from "./event.ts";
+import { TypedEventEmitter } from "./typed-event-emitter.ts";
+
+const logger = loggerInstance.getChild("RoomStickyEvents");
+
+export enum RoomStickyEventsEvent {
+    Update = "RoomStickyEvents.Update",
+}
+
+type StickyMatrixEvent = MatrixEvent & { unstableStickyExpiresAt: number };
+
+export type RoomStickyEventsMap = {
+    /**
+     * Fires when any sticky event changes happen in a room.
+     * @param added Any new sticky events with no predecessor events (matching sender, type, and sticky_key)
+     * @param updated Any sticky events that supersede an existing event (matching sender, type, and sticky_key)
+     * @param removed The events that were removed from the map due to expiry.
+     */
+    [RoomStickyEventsEvent.Update]: (
+        added: StickyMatrixEvent[],
+        updated: { current: StickyMatrixEvent; previous: StickyMatrixEvent }[],
+        removed: StickyMatrixEvent[],
+    ) => void;
+};
+
+/**
+ * Tracks sticky events on behalf of one room, and fires an event
+ * whenever a sticky event is updated or replaced.
+ */
+export class RoomStickyEventsStore extends TypedEventEmitter<RoomStickyEventsEvent, RoomStickyEventsMap> {
+    private readonly stickyEventsMap = new Map<string, Map<string, StickyMatrixEvent>>(); // (type -> stickyKey+userId) -> event
+    private readonly unkeyedStickyEvents = new Set<StickyMatrixEvent>();
+
+    private stickyEventTimer?: ReturnType<typeof setTimeout>;
+    private nextStickyEventExpiryTs: number = Number.MAX_SAFE_INTEGER;
+
+    /**
+     * Get all sticky events that are currently active.
+     * @returns An iterable set of events.
+     */
+    public *getStickyEvents(): Iterable<StickyMatrixEvent> {
+        yield* this.unkeyedStickyEvents;
+        for (const innerMap of this.stickyEventsMap.values()) {
+            yield* innerMap.values();
+        }
+    }
+
+    /**
+     * Get an active sticky event that match the given `type`, `sender`, and `stickyKey`
+     * @param type The event `type`.
+     * @param sender The sender of the sticky event.
+     * @param stickyKey The sticky key used by the event.
+     * @returns A matching active sticky event, or undefined.
+     */
+    public getKeyedStickyEvent(sender: string, type: string, stickyKey: string): StickyMatrixEvent | undefined {
+        return this.stickyEventsMap.get(type)?.get(`${stickyKey}${sender}`);
+    }
+
+    /**
+     * Get active sticky events without a sticky key that match the given `type` and `sender`.
+     * @param type The event `type`.
+     * @param sender The sender of the sticky event.
+     * @returns An array of matching sticky events.
+     */
+    public getUnkeyedStickyEvent(sender: string, type: string): StickyMatrixEvent[] {
+        return [...this.unkeyedStickyEvents].filter((ev) => ev.getType() === type && ev.getSender() === sender);
+    }
+
+    /**
+     * Adds a sticky event into the local sticky event map.
+     *
+     * NOTE: This will not cause `RoomEvent.StickyEvents` to be emitted.
+     *
+     * @throws If the `event` does not contain valid sticky data.
+     * @param event The MatrixEvent that contains sticky data.
+     * @returns An object describing whether the event was added to the map,
+     *          and the previous event it may have replaced.
+     */
+    private addStickyEvent(event: MatrixEvent): { added: true; prevEvent?: StickyMatrixEvent } | { added: false } {
+        const stickyKey = event.getContent().msc4354_sticky_key;
+        if (typeof stickyKey !== "string" && stickyKey !== undefined) {
+            throw new Error(`${event.getId()} is missing msc4354_sticky_key`);
+        }
+
+        // With this we have the guarantee, that all events in stickyEventsMap are correctly formatted
+        if (event.unstableStickyExpiresAt === undefined) {
+            throw new Error(`${event.getId()} is missing msc4354_sticky.duration_ms`);
+        }
+        const sender = event.getSender();
+        const type = event.getType();
+        if (!sender) {
+            throw new Error(`${event.getId()} is missing a sender`);
+        } else if (event.unstableStickyExpiresAt <= Date.now()) {
+            logger.info("ignored sticky event with older expiration time than current time", stickyKey);
+            return { added: false };
+        }
+
+        // While we fully expect the server to always provide the correct value,
+        // this is just insurance to protect against attacks on our Map.
+        if (!sender.startsWith("@")) {
+            throw new Error("Expected sender to start with @");
+        }
+
+        let prevEvent: StickyMatrixEvent | undefined;
+        if (stickyKey !== undefined) {
+            // Why this is safe:
+            // A type may contain anything but the *sender* is tightly
+            // constrained so that a key will always end with a @<user_id>
+            // E.g. Where a malicous event type might be "rtc.member.event@foo:bar" the key becomes:
+            // "rtc.member.event.@foo:bar@bar:baz"
+            const innerMapKey = `${stickyKey}${sender}`;
+            prevEvent = this.stickyEventsMap.get(type)?.get(innerMapKey);
+
+            // sticky events are not allowed to expire sooner than their predecessor.
+            if (prevEvent && event.unstableStickyExpiresAt < prevEvent.unstableStickyExpiresAt) {
+                logger.info("ignored sticky event with older expiry time", stickyKey);
+                return { added: false };
+            } else if (
+                prevEvent &&
+                event.getTs() === prevEvent.getTs() &&
+                (event.getId() ?? "") < (prevEvent.getId() ?? "")
+            ) {
+                // This path is unlikely, as it requires both events to have the same TS.
+                logger.info("ignored sticky event due to 'id tie break rule' on sticky_key", stickyKey);
+                return { added: false };
+            }
+            if (!this.stickyEventsMap.has(type)) {
+                this.stickyEventsMap.set(type, new Map());
+            }
+            this.stickyEventsMap.get(type)!.set(innerMapKey, event as StickyMatrixEvent);
+        } else {
+            this.unkeyedStickyEvents.add(event as StickyMatrixEvent);
+        }
+
+        // Recalculate the next expiry time.
+        this.nextStickyEventExpiryTs = Math.min(event.unstableStickyExpiresAt, this.nextStickyEventExpiryTs);
+
+        this.scheduleStickyTimer();
+        return { added: true, prevEvent };
+    }
+
+    /**
+     * Add a series of sticky events, emitting `RoomEvent.StickyEvents` if any
+     * changes were made.
+     * @param events A set of new sticky events.
+     */
+    public addStickyEvents(events: MatrixEvent[]): void {
+        const added: StickyMatrixEvent[] = [];
+        const updated: { current: StickyMatrixEvent; previous: StickyMatrixEvent }[] = [];
+        for (const event of events) {
+            try {
+                const result = this.addStickyEvent(event);
+                if (result.added) {
+                    if (result.prevEvent) {
+                        // e is validated as a StickyMatrixEvent by virtue of `addStickyEvent` returning added: true.
+                        updated.push({ current: event as StickyMatrixEvent, previous: result.prevEvent });
+                    } else {
+                        added.push(event as StickyMatrixEvent);
+                    }
+                }
+            } catch (ex) {
+                logger.warn("ignored invalid sticky event", ex);
+            }
+        }
+        if (added.length || updated.length) this.emit(RoomStickyEventsEvent.Update, added, updated, []);
+        this.scheduleStickyTimer();
+    }
+
+    /**
+     * Schedule the sticky event expiry timer. The timer will
+     * run immediately if an event has already expired.
+     */
+    private scheduleStickyTimer(): void {
+        if (this.stickyEventTimer) {
+            clearTimeout(this.stickyEventTimer);
+            this.stickyEventTimer = undefined;
+        }
+        if (this.nextStickyEventExpiryTs === Number.MAX_SAFE_INTEGER) {
+            // We have no events due to expire.
+            return;
+        } // otherwise, schedule in the future
+        this.stickyEventTimer = setTimeout(this.cleanExpiredStickyEvents, this.nextStickyEventExpiryTs - Date.now());
+    }
+
+    /**
+     * Clean out any expired sticky events.
+     */
+    private readonly cleanExpiredStickyEvents = (): void => {
+        const now = Date.now();
+        const removedEvents: StickyMatrixEvent[] = [];
+
+        // We will recalculate this as we check all events.
+        this.nextStickyEventExpiryTs = Number.MAX_SAFE_INTEGER;
+        for (const [eventType, innerEvents] of this.stickyEventsMap.entries()) {
+            for (const [innerMapKey, event] of innerEvents) {
+                // we only added items with `sticky` into this map so we can assert non-null here
+                if (now >= event.unstableStickyExpiresAt) {
+                    logger.debug("Expiring sticky event", event.getId());
+                    removedEvents.push(event);
+                    this.stickyEventsMap.get(eventType)!.delete(innerMapKey);
+                } else {
+                    // If not removing the event, check to see if it's the next lowest expiry.
+                    this.nextStickyEventExpiryTs = Math.min(
+                        this.nextStickyEventExpiryTs,
+                        event.unstableStickyExpiresAt,
+                    );
+                }
+            }
+            // Clean up map after use.
+            if (this.stickyEventsMap.get(eventType)?.size === 0) {
+                this.stickyEventsMap.delete(eventType);
+            }
+        }
+        for (const event of this.unkeyedStickyEvents) {
+            if (now >= event.unstableStickyExpiresAt) {
+                logger.debug("Expiring sticky event", event.getId());
+                this.unkeyedStickyEvents.delete(event);
+                removedEvents.push(event);
+            } else {
+                // If not removing the event, check to see if it's the next lowest expiry.
+                this.nextStickyEventExpiryTs = Math.min(this.nextStickyEventExpiryTs, event.unstableStickyExpiresAt);
+            }
+        }
+        if (removedEvents.length) {
+            this.emit(RoomStickyEventsEvent.Update, [], [], removedEvents);
+        }
+        // Finally, schedule the next run.
+        this.scheduleStickyTimer();
+    };
+
+    /**
+     * Clear all events and stop the timer from firing.
+     */
+    public clear(): void {
+        this.stickyEventsMap.clear();
+        // Unschedule timer.
+        this.nextStickyEventExpiryTs = Number.MAX_SAFE_INTEGER;
+        this.scheduleStickyTimer();
+    }
+}
diff --git a/src/models/room.ts b/src/models/room.ts
index 6cdfaa39a7..ffd35c2a7d 100644
--- a/src/models/room.ts
+++ b/src/models/room.ts
@@ -77,6 +77,7 @@ import { compareEventOrdering } from "./compare-event-ordering.ts";
 import { KnownMembership, type Membership } from "../@types/membership.ts";
 import { type Capabilities, type IRoomVersionsCapability, RoomVersionStability } from "../serverCapabilities.ts";
 import { type MSC4186Hero } from "../sliding-sync.ts";
+import { RoomStickyEventsStore, RoomStickyEventsEvent, type RoomStickyEventsMap } from "./room-sticky-events.ts";
 
 // These constants are used as sane defaults when the homeserver doesn't support
 // the m.room_versions capability. In practice, KNOWN_SAFE_ROOM_VERSION should be
@@ -167,6 +168,7 @@ export type RoomEmittedEvents =
     | RoomStateEvent.NewMember
     | RoomStateEvent.Update
     | RoomStateEvent.Marker
+    | RoomStickyEventsEvent.Update
     | ThreadEvent.New
     | ThreadEvent.Update
     | ThreadEvent.NewReply
@@ -320,6 +322,7 @@ export type RoomEventHandlerMap = {
 } & Pick<ThreadHandlerMap, ThreadEvent.Update | ThreadEvent.NewReply | ThreadEvent.Delete> &
     EventTimelineSetHandlerMap &
     Pick<MatrixEventHandlerMap, MatrixEventEvent.BeforeRedaction> &
+    Pick<RoomStickyEventsMap, RoomStickyEventsEvent.Update> &
     Pick<
         RoomStateEventHandlerMap,
         | RoomStateEvent.Events
@@ -446,6 +449,11 @@ export class Room extends ReadReceipt<RoomEmittedEvents, RoomEventHandlerMap> {
      */
     private roomReceipts = new RoomReceipts(this);
 
+    /**
+     * Stores and tracks sticky events
+     */
+    private stickyEvents = new RoomStickyEventsStore();
+
     /**
      * Construct a new Room.
      *
@@ -492,6 +500,7 @@ export class Room extends ReadReceipt<RoomEmittedEvents, RoomEventHandlerMap> {
         // Listen to our own receipt event as a more modular way of processing our own
         // receipts. No need to remove the listener: it's on ourself anyway.
         this.on(RoomEvent.Receipt, this.onReceipt);
+        this.reEmitter.reEmit(this.stickyEvents, [RoomStickyEventsEvent.Update]);
 
         // all our per-room timeline sets. the first one is the unfiltered ones;
         // the subsequent ones are the filtered ones in no particular order.
@@ -3414,6 +3423,55 @@ export class Room extends ReadReceipt<RoomEmittedEvents, RoomEventHandlerMap> {
         return this.accountData.get(type);
     }
 
+    /**
+     * Get an iterator of currently active sticky events.
+     */
+    // eslint-disable-next-line
+    public _unstable_getStickyEvents(): ReturnType<RoomStickyEventsStore["getStickyEvents"]> {
+        return this.stickyEvents.getStickyEvents();
+    }
+
+    /**
+     * Get a sticky event that match the given `type`, `sender`, and `stickyKey`
+     * @param type The event `type`.
+     * @param sender The sender of the sticky event.
+     * @param stickyKey The sticky key used by the event.
+     * @returns A matching active sticky event, or undefined.
+     */
+    // eslint-disable-next-line
+    public _unstable_getKeyedStickyEvent(
+        sender: string,
+        type: string,
+        stickyKey: string,
+    ): ReturnType<RoomStickyEventsStore["getKeyedStickyEvent"]> {
+        return this.stickyEvents.getKeyedStickyEvent(sender, type, stickyKey);
+    }
+
+    /**
+     * Get active sticky events without a sticky key that match the given `type` and `sender`.
+     * @param type The event `type`.
+     * @param sender The sender of the sticky event.
+     * @returns An array of matching sticky events.
+     */
+    // eslint-disable-next-line
+    public _unstable_getUnkeyedStickyEvent(
+        sender: string,
+        type: string,
+    ): ReturnType<RoomStickyEventsStore["getUnkeyedStickyEvent"]> {
+        return this.stickyEvents.getUnkeyedStickyEvent(sender, type);
+    }
+
+    /**
+     * Add a series of sticky events, emitting `RoomEvent.StickyEvents` if any
+     * changes were made.
+     * @param events A set of new sticky events.
+     * @internal
+     */
+    // eslint-disable-next-line
+    public _unstable_addStickyEvents(events: MatrixEvent[]): ReturnType<RoomStickyEventsStore["addStickyEvents"]> {
+        return this.stickyEvents.addStickyEvents(events);
+    }
+
     /**
      * Returns whether the syncing user has permission to send a message in the room
      * @returns true if the user should be permitted to send
diff --git a/src/sync-accumulator.ts b/src/sync-accumulator.ts
index 816b45de7e..28b06870e3 100644
--- a/src/sync-accumulator.ts
+++ b/src/sync-accumulator.ts
@@ -20,7 +20,7 @@ limitations under the License.
 
 import { logger } from "./logger.ts";
 import { deepCopy } from "./utils.ts";
-import { type IContent, type IUnsigned } from "./models/event.ts";
+import { MAX_STICKY_DURATION_MS, type IContent, type IUnsigned } from "./models/event.ts";
 import { type IRoomSummary } from "./models/room-summary.ts";
 import { type EventType } from "./@types/event.ts";
 import { UNREAD_THREAD_NOTIFICATIONS } from "./@types/sync.ts";
@@ -76,11 +76,25 @@ export interface ITimeline {
     prev_batch: string | null;
 }
 
+type StickyEventFields = {
+    msc4354_sticky: { duration_ms: number };
+    content: { msc4354_sticky_key?: string };
+};
+
+export type IStickyEvent = IRoomEvent & StickyEventFields;
+
+export type IStickyStateEvent = IStateEvent & StickyEventFields;
+
+export interface ISticky {
+    events: Array<IStickyEvent | IStickyStateEvent>;
+}
+
 export interface IJoinedRoom {
     "summary": IRoomSummary;
     // One of `state` or `state_after` is required.
     "state"?: IState;
     "org.matrix.msc4222.state_after"?: IState; // https://github.com/matrix-org/matrix-spec-proposals/pull/4222
+    "msc4354_sticky"?: ISticky; // https://github.com/matrix-org/matrix-spec-proposals/pull/4354
     "timeline": ITimeline;
     "ephemeral": IEphemeral;
     "account_data": IAccountData;
@@ -201,6 +215,14 @@ interface IRoom {
     _unreadNotifications: Partial<UnreadNotificationCounts>;
     _unreadThreadNotifications?: Record<string, Partial<UnreadNotificationCounts>>;
     _receipts: ReceiptAccumulator;
+    _stickyEvents: {
+        readonly event: IStickyEvent | IStickyStateEvent;
+        /**
+         * This is the timestamp at which point it is safe to remove this event from the store.
+         * This value is immutable
+         */
+        readonly expiresTs: number;
+    }[];
 }
 
 export interface ISyncData {
@@ -411,6 +433,7 @@ export class SyncAccumulator {
 
     // Accumulate timeline and state events in a room.
     private accumulateJoinState(roomId: string, data: IJoinedRoom, fromDatabase = false): void {
+        const now = Date.now();
         // We expect this function to be called a lot (every /sync) so we want
         // this to be fast. /sync stores events in an array but we often want
         // to clobber based on type/state_key. Rather than convert arrays to
@@ -457,6 +480,7 @@ export class SyncAccumulator {
                 _unreadThreadNotifications: {},
                 _summary: {},
                 _receipts: new ReceiptAccumulator(),
+                _stickyEvents: [],
             };
         }
         const currentData = this.joinRooms[roomId];
@@ -540,6 +564,27 @@ export class SyncAccumulator {
             });
         });
 
+        // Prune out any events in our stores that have since expired, do this before we
+        // insert new events.
+        currentData._stickyEvents = currentData._stickyEvents.filter(({ expiresTs }) => expiresTs > now);
+
+        // We want this to be fast, so don't worry about duplicate events here. The RoomStickyEventsStore will
+        // process these events into the correct mapped order.
+        if (data.msc4354_sticky?.events) {
+            currentData._stickyEvents = currentData._stickyEvents.concat(
+                data.msc4354_sticky.events.map((event) => {
+                    // If `duration_ms` exceeds the spec limit of a hour, we cap it.
+                    const cappedDuration = Math.min(event.msc4354_sticky.duration_ms, MAX_STICKY_DURATION_MS);
+                    // If `origin_server_ts` claims to have been from the future, we still bound it to now.
+                    const createdTs = Math.min(event.origin_server_ts, now);
+                    return {
+                        event,
+                        expiresTs: cappedDuration + createdTs,
+                    };
+                }),
+            );
+        }
+
         // attempt to prune the timeline by jumping between events which have
         // pagination tokens.
         if (currentData._timeline.length > this.opts.maxTimelineEntries!) {
@@ -611,6 +656,11 @@ export class SyncAccumulator {
                 "unread_notifications": roomData._unreadNotifications,
                 "unread_thread_notifications": roomData._unreadThreadNotifications,
                 "summary": roomData._summary as IRoomSummary,
+                "msc4354_sticky": roomData._stickyEvents?.length
+                    ? {
+                          events: roomData._stickyEvents.map((e) => e.event),
+                      }
+                    : undefined,
             };
             // Add account data
             Object.keys(roomData._accountData).forEach((evType) => {
diff --git a/src/sync.ts b/src/sync.ts
index 4cc23c0a18..a191fa8776 100644
--- a/src/sync.ts
+++ b/src/sync.ts
@@ -1082,6 +1082,8 @@ export class SyncApi {
         //              highlight_count: 0,
         //              notification_count: 0,
         //          }
+        //          "org.matrix.msc4222.state_after": { events: [] },  // only if "org.matrix.msc4222.use_state_after" is true
+        //          msc4354_sticky: { events: [] }, // only if "org.matrix.msc4354.sticky" is true
         //        }
         //      },
         //      leave: {
@@ -1219,6 +1221,7 @@ export class SyncApi {
             const timelineEvents = this.mapSyncEventsFormat(joinObj.timeline, room, false);
             const ephemeralEvents = this.mapSyncEventsFormat(joinObj.ephemeral);
             const accountDataEvents = this.mapSyncEventsFormat(joinObj.account_data);
+            const stickyEvents = this.mapSyncEventsFormat(joinObj.msc4354_sticky);
 
             // If state_after is present, this is the events that form the state at the end of the timeline block and
             // regular timeline events do *not* count towards state. If it's not present, then the state is formed by
@@ -1402,6 +1405,18 @@ export class SyncApi {
             // we deliberately don't add accountData to the timeline
             room.addAccountData(accountDataEvents);
 
+            // Sticky events primarily come via the `timeline` field, with the
+            // sticky info field marking them as sticky.
+            // If the sync is "gappy" (meaning it is skipping events to catch up) then
+            // sticky events will instead come down the sticky section.
+            // This ensures we collect sticky events from both places.
+            const stickyEventsAndStickyEventsFromTheTimeline = stickyEvents.concat(
+                timelineEvents.filter((e) => e.unstableStickyInfo !== undefined),
+            );
+            // Note: We calculate sticky events before emitting `.Room` as it's nice to have
+            // sticky events calculated and ready to go.
+            room._unstable_addStickyEvents(stickyEventsAndStickyEventsFromTheTimeline);
+
             room.recalculate();
             if (joinObj.isBrandNewRoom) {
                 client.store.storeRoom(room);
@@ -1411,11 +1426,21 @@ export class SyncApi {
             this.processEventsForNotifs(room, timelineEvents);
 
             const emitEvent = (e: MatrixEvent): boolean => client.emit(ClientEvent.Event, e);
+            // this fires a couple of times for some events. (eg state events are in the timeline and the state)
+            // should this get a sync section as an additional event emission param (e, syncSection))?
             stateEvents.forEach(emitEvent);
             timelineEvents.forEach(emitEvent);
             ephemeralEvents.forEach(emitEvent);
             accountDataEvents.forEach(emitEvent);
-
+            stickyEvents
+                .filter(
+                    (stickyEvent) =>
+                        // This is highly unlikey, but in the case where a sticky event
+                        // has appeared in the timeline AND the sticky section, we only
+                        // want to emit the event once.
+                        !timelineEvents.some((timelineEvent) => timelineEvent.getId() === stickyEvent.getId()),
+                )
+                .forEach(emitEvent);
             // Decrypt only the last message in all rooms to make sure we can generate a preview
             // And decrypt all events after the recorded read receipt to ensure an accurate
             // notification count