diff --git a/CHANGELOG.md b/CHANGELOG.md
index f236cca1f..d866f221e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -19,6 +19,7 @@ and this project adheres to
 - ✨(frontend) allow group members filtering #363
 - ✨(mailbox) send new mailbox confirmation email #397
 - ✨(domains) domain accesses update API #423
+- ✨(backend) domain accesses create API #428
 
 ### Fixed
 
diff --git a/src/backend/mailbox_manager/api/serializers.py b/src/backend/mailbox_manager/api/serializers.py
index 76a784b3e..25fea76e8 100644
--- a/src/backend/mailbox_manager/api/serializers.py
+++ b/src/backend/mailbox_manager/api/serializers.py
@@ -5,8 +5,9 @@
 from rest_framework import exceptions, serializers
 
 from core.api.serializers import UserSerializer
+from core.models import User
 
-from mailbox_manager import models
+from mailbox_manager import enums, models
 from mailbox_manager.utils.dimail import DimailAPIClient
 
 
@@ -85,13 +86,13 @@ class MailDomainAccessSerializer(serializers.ModelSerializer):
 
     class Meta:
         model = models.MailDomainAccess
-        fields = [
-            "id",
-            "user",
-            "role",
-            "can_set_role_to",
-        ]
-        read_only_fields = ["id", "user", "can_set_role_to"]
+        fields = ["id", "user", "role", "can_set_role_to"]
+        read_only_fields = ["id", "can_set_role_to"]
+
+    def update(self, instance, validated_data):
+        """Make "user" field is readonly but only on update."""
+        validated_data.pop("user", None)
+        return super().update(instance, validated_data)
 
     def get_can_set_role_to(self, access):
         """Return roles available to set for the authenticated user"""
@@ -99,8 +100,9 @@ def get_can_set_role_to(self, access):
 
     def validate(self, attrs):
         """
-        Check access rights specific to writing (update)
+        Check access rights specific to writing (update/create)
         """
+
         request = self.context.get("request")
         authenticated_user = getattr(request, "user", None)
         role = attrs.get("role")
@@ -116,6 +118,45 @@ def validate(self, attrs):
                     else "You are not allowed to modify role for this user."
                 )
                 raise exceptions.PermissionDenied(message)
+        # Create
+        else:
+            # A domain slug has to be set to create a new access
+            try:
+                domain_slug = self.context["domain_slug"]
+            except KeyError as exc:
+                raise exceptions.ValidationError(
+                    "You must set a domain slug in kwargs to create a new domain access."
+                ) from exc
+
+            try:
+                access = authenticated_user.mail_domain_accesses.get(
+                    domain__slug=domain_slug
+                )
+            except models.MailDomainAccess.DoesNotExist as exc:
+                raise exceptions.PermissionDenied(
+                    "You are not allowed to manage accesses for this domain."
+                ) from exc
+
+            # Authenticated user must be owner or admin of current domain to set new roles
+            if access.role not in [
+                enums.MailDomainRoleChoices.OWNER,
+                enums.MailDomainRoleChoices.ADMIN,
+            ]:
+                raise exceptions.PermissionDenied(
+                    "You are not allowed to manage accesses for this domain."
+                )
+            # only an owner can set an owner role to another user
+            if (
+                role == enums.MailDomainRoleChoices.OWNER
+                and access.role != enums.MailDomainRoleChoices.OWNER
+            ):
+                raise exceptions.PermissionDenied(
+                    "Only owners of a domain can assign other users as owners."
+                )
+            attrs["user"] = User.objects.get(pk=self.initial_data["user"])
+            attrs["domain"] = models.MailDomain.objects.get(
+                slug=self.context["domain_slug"]
+            )
         return attrs
 
 
diff --git a/src/backend/mailbox_manager/api/viewsets.py b/src/backend/mailbox_manager/api/viewsets.py
index 37399a886..b2fcbb67f 100644
--- a/src/backend/mailbox_manager/api/viewsets.py
+++ b/src/backend/mailbox_manager/api/viewsets.py
@@ -58,6 +58,7 @@ def perform_create(self, serializer):
 class MailDomainAccessViewSet(
     viewsets.GenericViewSet,
     mixins.ListModelMixin,
+    mixins.CreateModelMixin,
     mixins.UpdateModelMixin,
     mixins.RetrieveModelMixin,
 ):
@@ -68,6 +69,11 @@ class MailDomainAccessViewSet(
         Return list of all domain accesses related to the logged-in user and one
         domain access if an id is provided.
 
+    POST /api/v1.0/mail-domains/<domain_slug>/accesses/ with expected data:
+        - user: str
+        - role: str [owner|admin|viewer]
+        Return newly created mail domain access
+
     PUT /api/v1.0/mail-domains/<domain_slug>/accesses/<domain_access_id>/ with expected data:
         - role: str [owner|admin|viewer]
         Return updated domain access
diff --git a/src/backend/mailbox_manager/tests/api/mail_domain_accesses/test_api_mail_domain_accesses_create.py b/src/backend/mailbox_manager/tests/api/mail_domain_accesses/test_api_mail_domain_accesses_create.py
new file mode 100644
index 000000000..46a3bff5a
--- /dev/null
+++ b/src/backend/mailbox_manager/tests/api/mail_domain_accesses/test_api_mail_domain_accesses_create.py
@@ -0,0 +1,173 @@
+"""
+Test for mail domain accesses API endpoints in People's core app : create
+"""
+
+import random
+
+import pytest
+from rest_framework import status
+from rest_framework.test import APIClient
+
+from core import factories as core_factories
+
+from mailbox_manager import enums, factories, models
+
+pytestmark = pytest.mark.django_db
+
+
+def test_api_mail_domain__accesses_create_anonymous():
+    """Anonymous users should not be allowed to create mail domain accesses."""
+    user = core_factories.UserFactory()
+    mail_domain = factories.MailDomainFactory()
+    for role in [role[0] for role in enums.MailDomainRoleChoices.choices]:
+        response = APIClient().post(
+            f"/api/v1.0/mail-domains/{mail_domain.slug}/accesses/",
+            {
+                "user": str(user.id),
+                "role": role,
+            },
+            format="json",
+        )
+        assert response.status_code == status.HTTP_401_UNAUTHORIZED
+        assert response.json() == {
+            "detail": "Authentication credentials were not provided."
+        }
+        assert models.MailDomainAccess.objects.exists() is False
+
+
+def test_api_mail_domain__accesses_create_authenticated_unrelated():
+    """
+    Authenticated users should not be allowed to create domain accesses for a domain to
+    which they are not related.
+    """
+    user = core_factories.UserFactory()
+    other_user = core_factories.UserFactory()
+    domain = factories.MailDomainFactory()
+
+    client = APIClient()
+    client.force_login(user)
+    for role in [role[0] for role in enums.MailDomainRoleChoices.choices]:
+        response = client.post(
+            f"/api/v1.0/mail-domains/{domain.slug}/accesses/",
+            {
+                "user": str(other_user.id),
+                "role": role,
+            },
+            format="json",
+        )
+
+        assert response.status_code == status.HTTP_403_FORBIDDEN
+        assert response.json() == {
+            "detail": "You are not allowed to manage accesses for this domain."
+        }
+        assert not models.MailDomainAccess.objects.filter(user=other_user).exists()
+
+
+def test_api_mail_domain__accesses_create_authenticated_viewer():
+    """Viewer of a mail domain should not be allowed to create mail domain accesses."""
+    authenticated_user = core_factories.UserFactory()
+    mail_domain = factories.MailDomainFactory(
+        users=[(authenticated_user, enums.MailDomainRoleChoices.VIEWER)]
+    )
+    other_user = core_factories.UserFactory()
+
+    client = APIClient()
+    client.force_login(authenticated_user)
+    for role in [role[0] for role in enums.MailDomainRoleChoices.choices]:
+        response = client.post(
+            f"/api/v1.0/mail-domains/{mail_domain.slug}/accesses/",
+            {
+                "user": str(other_user.id),
+                "role": role,
+            },
+            format="json",
+        )
+
+        assert response.status_code == status.HTTP_403_FORBIDDEN
+        assert response.json() == {
+            "detail": "You are not allowed to manage accesses for this domain."
+        }
+
+    assert not models.MailDomainAccess.objects.filter(user=other_user).exists()
+
+
+def test_api_mail_domain__accesses_create_authenticated_administrator():
+    """
+    Administrators of a domain should be able to create mail domain accesses
+    except for the "owner" role.
+    """
+    authenticated_user = core_factories.UserFactory()
+    mail_domain = factories.MailDomainFactory(
+        users=[(authenticated_user, enums.MailDomainRoleChoices.ADMIN)]
+    )
+    other_user = core_factories.UserFactory()
+
+    client = APIClient()
+    client.force_login(authenticated_user)
+
+    # It should not be allowed to create an owner access
+    response = client.post(
+        f"/api/v1.0/mail-domains/{mail_domain.slug}/accesses/",
+        {
+            "user": str(other_user.id),
+            "role": enums.MailDomainRoleChoices.OWNER,
+        },
+        format="json",
+    )
+
+    assert response.status_code == status.HTTP_403_FORBIDDEN
+    assert response.json() == {
+        "detail": "Only owners of a domain can assign other users as owners."
+    }
+
+    # It should be allowed to create a lower access
+    for role in [enums.MailDomainRoleChoices.ADMIN, enums.MailDomainRoleChoices.VIEWER]:
+        other_user = core_factories.UserFactory()
+        response = client.post(
+            f"/api/v1.0/mail-domains/{mail_domain.slug}/accesses/",
+            {
+                "user": str(other_user.id),
+                "role": role,
+            },
+            format="json",
+        )
+        assert response.status_code == status.HTTP_201_CREATED
+        new_mail_domain_access = models.MailDomainAccess.objects.filter(
+            user=other_user
+        ).last()
+
+        assert response.json()["id"] == str(new_mail_domain_access.id)
+        assert response.json()["role"] == role
+    assert models.MailDomainAccess.objects.filter(domain=mail_domain).count() == 3
+
+
+def test_api_mail_domain__accesses_create_authenticated_owner():
+    """
+    Owners of a mail domain should be able to create mail domain accesses whatever the role.
+    """
+    authenticated_user = core_factories.UserFactory()
+    mail_domain = factories.MailDomainFactory(
+        users=[(authenticated_user, enums.MailDomainRoleChoices.OWNER)]
+    )
+    other_user = core_factories.UserFactory()
+
+    role = random.choice([role[0] for role in enums.MailDomainRoleChoices.choices])
+
+    client = APIClient()
+    client.force_login(authenticated_user)
+    response = client.post(
+        f"/api/v1.0/mail-domains/{mail_domain.slug}/accesses/",
+        {
+            "user": str(other_user.id),
+            "role": role,
+        },
+        format="json",
+    )
+
+    assert response.status_code == status.HTTP_201_CREATED
+    assert models.MailDomainAccess.objects.filter(user=other_user).count() == 1
+    new_mail_domain_access = models.MailDomainAccess.objects.filter(
+        user=other_user
+    ).get()
+    assert response.json()["id"] == str(new_mail_domain_access.id)
+    assert response.json()["role"] == role