diff --git a/src/event-handler/index.ts b/src/event-handler/index.ts
index ea6bd98d..4a82eeb9 100644
--- a/src/event-handler/index.ts
+++ b/src/event-handler/index.ts
@@ -1,4 +1,4 @@
-import { createLogger } from "../createLogger";
+import { createLogger } from "../createLogger.js";
 import type {
   EmitterWebhookEvent,
   EmitterWebhookEventName,
@@ -11,9 +11,9 @@ import {
   receiverOn as on,
   receiverOnAny as onAny,
   receiverOnError as onError,
-} from "./on";
-import { receiverHandle as receive } from "./receive";
-import { removeListener } from "./remove-listener";
+} from "./on.js";
+import { receiverHandle as receive } from "./receive.js";
+import { removeListener } from "./remove-listener.js";
 
 interface EventHandler<TTransformed> {
   on<E extends EmitterWebhookEventName>(
diff --git a/src/event-handler/on.ts b/src/event-handler/on.ts
index e95e6216..6e4a0399 100644
--- a/src/event-handler/on.ts
+++ b/src/event-handler/on.ts
@@ -1,4 +1,4 @@
-import { emitterEventNames } from "../generated/webhook-names";
+import { emitterEventNames } from "../generated/webhook-names.js";
 import type {
   EmitterWebhookEvent,
   EmitterWebhookEventName,
diff --git a/src/index.ts b/src/index.ts
index 373cb2a6..6a0e4125 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,7 +1,7 @@
-import { createLogger } from "./createLogger";
-import { createEventHandler } from "./event-handler/index";
+import { createLogger } from "./createLogger.js";
+import { createEventHandler } from "./event-handler/index.js";
 import { sign, verify } from "@octokit/webhooks-methods";
-import { verifyAndReceive } from "./verify-and-receive";
+import { verifyAndReceive } from "./verify-and-receive.js";
 import type {
   EmitterWebhookEvent,
   EmitterWebhookEventName,
@@ -12,10 +12,10 @@ import type {
   WebhookError,
   WebhookEventHandlerError,
   EmitterWebhookEventWithStringPayloadAndSignature,
-} from "./types";
+} from "./types.ts";
 
-export { createNodeMiddleware } from "./middleware/node/index";
-export { emitterEventNames } from "./generated/webhook-names";
+export { createNodeMiddleware } from "./middleware/node/index.js";
+export { emitterEventNames } from "./generated/webhook-names.js";
 
 // U holds the return value of `transform` function in Options
 class Webhooks<TTransformed = unknown> {
diff --git a/src/middleware/node/index.ts b/src/middleware/node/index.ts
index 7f57368d..a3aacd37 100644
--- a/src/middleware/node/index.ts
+++ b/src/middleware/node/index.ts
@@ -1,7 +1,7 @@
-import { createLogger } from "../../createLogger";
-import type { Webhooks } from "../../index";
-import { middleware } from "./middleware";
-import type { MiddlewareOptions } from "./types";
+import { createLogger } from "../../createLogger.js";
+import type { Webhooks } from "../../index.js";
+import { middleware } from "./middleware.js";
+import type { MiddlewareOptions } from "./types.ts";
 
 export function createNodeMiddleware(
   webhooks: Webhooks,
diff --git a/src/middleware/node/types.ts b/src/middleware/node/types.ts
index 92dec00f..a7138589 100644
--- a/src/middleware/node/types.ts
+++ b/src/middleware/node/types.ts
@@ -1,4 +1,4 @@
-import type { Logger } from "../../createLogger";
+import type { Logger } from "../../createLogger.js";
 
 export type MiddlewareOptions = {
   path?: string;
diff --git a/test/integration/event-handler-test.ts b/test/integration/event-handler-test.ts
index 67ec266c..d03b30e1 100644
--- a/test/integration/event-handler-test.ts
+++ b/test/integration/event-handler-test.ts
@@ -1,6 +1,9 @@
-import { createEventHandler } from "../../src/event-handler";
+import { createEventHandler } from "../../src/event-handler/index.js";
 import { EmitterWebhookEvent, WebhookEventHandlerError } from "../../src/types";
-import { installationCreatedPayload, pushEventPayload } from "../fixtures";
+import {
+  installationCreatedPayload,
+  pushEventPayload,
+} from "../fixtures/index.js";
 
 test("events", async () => {
   const eventHandler = createEventHandler({});
diff --git a/test/integration/node-middleware.test.ts b/test/integration/node-middleware.test.ts
index e2500807..71fba234 100644
--- a/test/integration/node-middleware.test.ts
+++ b/test/integration/node-middleware.test.ts
@@ -6,7 +6,7 @@ import { sign } from "@octokit/webhooks-methods";
 // import without types
 const express = require("express");
 
-import { createNodeMiddleware, Webhooks } from "../../src";
+import { createNodeMiddleware, Webhooks } from "../../src/index.ts";
 
 const pushEventPayload = readFileSync(
   "test/fixtures/push-payload.json",
diff --git a/test/integration/smoke-test.ts b/test/integration/smoke-test.ts
index 987290dc..1c763464 100644
--- a/test/integration/smoke-test.ts
+++ b/test/integration/smoke-test.ts
@@ -1,4 +1,4 @@
-import { Webhooks, createEventHandler, emitterEventNames } from "../../src";
+import { Webhooks, createEventHandler, emitterEventNames } from "../../src/index.ts";
 
 test("@octokit/webhooks", () => {
   const emitWarningSpy = jest.spyOn(process, "emitWarning");
diff --git a/test/integration/webhooks.test.ts b/test/integration/webhooks.test.ts
index 9973a55a..04db8d48 100644
--- a/test/integration/webhooks.test.ts
+++ b/test/integration/webhooks.test.ts
@@ -2,7 +2,7 @@ import { readFileSync } from "fs";
 
 import { sign } from "@octokit/webhooks-methods";
 
-import { Webhooks } from "../../src";
+import { Webhooks } from "../../src/index.ts";
 
 const pushEventPayloadString = readFileSync(
   "test/fixtures/push-payload.json",
diff --git a/test/typescript-validate.ts b/test/typescript-validate.ts
index db7249c6..94c6234e 100644
--- a/test/typescript-validate.ts
+++ b/test/typescript-validate.ts
@@ -4,7 +4,7 @@ import {
   EmitterWebhookEvent,
   WebhookError,
   createNodeMiddleware,
-} from "../src/index";
+} from "../src/index.js";
 import { createServer } from "http";
 import {
   HandlerFunction,
diff --git a/test/unit/createLogger-test.ts b/test/unit/createLogger-test.ts
index 568a6fde..1c186302 100644
--- a/test/unit/createLogger-test.ts
+++ b/test/unit/createLogger-test.ts
@@ -1,4 +1,4 @@
-import { createLogger } from "../../src/createLogger";
+import { createLogger } from "../../src/createLogger.js";
 
 const noop = () => {};
 
diff --git a/test/unit/event-handler-on-test.ts b/test/unit/event-handler-on-test.ts
index 61357a2e..604b9232 100644
--- a/test/unit/event-handler-on-test.ts
+++ b/test/unit/event-handler-on-test.ts
@@ -1,4 +1,4 @@
-import { receiverOn } from "../../src/event-handler/on";
+import { receiverOn } from "../../src/event-handler/on.js";
 import { State } from "../../src/types";
 
 function noop() {}
diff --git a/test/unit/event-handler-receive-test.ts b/test/unit/event-handler-receive-test.ts
index 240566eb..224c475c 100644
--- a/test/unit/event-handler-receive-test.ts
+++ b/test/unit/event-handler-receive-test.ts
@@ -1,4 +1,4 @@
-import { receiverHandle as receive } from "../../src/event-handler/receive";
+import { receiverHandle as receive } from "../../src/event-handler/receive.js";
 import { State } from "../../src/types";
 
 const state: State = {
diff --git a/test/unit/event-handler-remove-listener-test.ts b/test/unit/event-handler-remove-listener-test.ts
index 231be656..8fec9d94 100644
--- a/test/unit/event-handler-remove-listener-test.ts
+++ b/test/unit/event-handler-remove-listener-test.ts
@@ -1,4 +1,4 @@
-import { removeListener } from "../../src/event-handler/remove-listener";
+import { removeListener } from "../../src/event-handler/remove-listener.js";
 import { State } from "../../src/types";
 
 test("remove-listener: single listener", () => {
diff --git a/test/unit/event-handler-wrap-error-handler-test.ts b/test/unit/event-handler-wrap-error-handler-test.ts
index 63b8ecf2..1cfafb96 100644
--- a/test/unit/event-handler-wrap-error-handler-test.ts
+++ b/test/unit/event-handler-wrap-error-handler-test.ts
@@ -1,4 +1,4 @@
-import { wrapErrorHandler } from "../../src/event-handler/wrap-error-handler";
+import { wrapErrorHandler } from "../../src/event-handler/wrap-error-handler.js";
 
 const noop = () => {};