diff --git a/changelog.d/20241.bugfix b/changelog.d/20241.bugfix
new file mode 100644
index 0000000000..3478b28dd3
--- /dev/null
+++ b/changelog.d/20241.bugfix
@@ -0,0 +1 @@
+Reject user creation via the Admin API when Synapse is delegating authentication to MAS.
diff --git a/synapse/rest/admin/users.py b/synapse/rest/admin/users.py
index 43dab16598..ecf7fc45f4 100644
--- a/synapse/rest/admin/users.py
+++ b/synapse/rest/admin/users.py
@@ -237,6 +237,7 @@ class UserRestServletV2Get(RestServlet):
         self.pusher_pool = hs.get_pusherpool()
         self._msc3866_enabled = hs.config.experimental.msc3866.enabled
         self._all_user_types = hs.config.user_types.all_user_types
+        self._auth_delegation_enabled = hs.config.mas.enabled
 
     async def on_GET(
         self, request: SynapseRequest, user_id: str
@@ -479,6 +480,15 @@ class UserRestServletV2(UserRestServletV2Get):
             return HTTPStatus.OK, user
 
         else:  # create user
+            if self._auth_delegation_enabled:
+                raise SynapseError(
+                    HTTPStatus.FORBIDDEN,
+                    "User creation via the Admin API is not available when "
+                    "Synapse is delegating authentication to MAS. Create the "
+                    "user via MAS instead.",
+                    errcode=Codes.FORBIDDEN,
+                )
+
             displayname = body.get("displayname", None)
 
             password_hash = None
diff --git a/tests/handlers/test_oauth_delegation.py b/tests/handlers/test_oauth_delegation.py
index 0fc1e17d6b..d37a58949e 100644
--- a/tests/handlers/test_oauth_delegation.py
+++ b/tests/handlers/test_oauth_delegation.py
@@ -662,3 +662,43 @@ class DisabledEndpointsTestCase(HomeserverTestCase):
         self.expect_unrecognized("GET", "/_synapse/admin/v1/users/foo/admin")
         self.expect_unrecognized("PUT", "/_synapse/admin/v1/users/foo/admin")
         self.expect_unrecognized("POST", "/_synapse/admin/v1/account_validity/validity")
+
+    def _mock_admin_requester(self) -> None:
+        self.hs.get_auth().get_user_by_req = AsyncMock(  # type: ignore[method-assign]
+            return_value=create_requester(user_id=USER_ID, device_id=DEVICE)
+        )
+        self.hs.get_auth().is_server_admin = AsyncMock(return_value=True)  # type: ignore[method-assign]
+
+    def test_admin_api_create_user_rejected(self) -> None:
+        """MAS never learns about a user created through the admin API, so the
+        account would be unusable."""
+        self._mock_admin_requester()
+
+        channel = self.make_request(
+            "PUT",
+            "/_synapse/admin/v2/users/@newuser:test",
+            {"password": "hunter2"},
+            access_token="token",
+        )
+
+        self.assertEqual(channel.code, 403, channel.json_body)
+        self.assertEqual(
+            channel.json_body["errcode"], Codes.FORBIDDEN, channel.json_body
+        )
+
+    def test_admin_api_modify_existing_user_allowed(self) -> None:
+        """Only creation is rejected: modifying a user MAS already knows about
+        has to keep working, since MAS has no equivalent endpoint."""
+        self._mock_admin_requester()
+        self.get_success(
+            self.hs.get_datastores().main.register_user(f"@existing:{SERVER_NAME}")
+        )
+
+        channel = self.make_request(
+            "PUT",
+            "/_synapse/admin/v2/users/@existing:test",
+            {"displayname": "Existing User"},
+            access_token="token",
+        )
+
+        self.assertEqual(channel.code, 200, channel.json_body)