diff --git a/changelog.d/11615.misc b/changelog.d/11615.misc
new file mode 100644
index 000000000000..0aa953650493
--- /dev/null
+++ b/changelog.d/11615.misc
@@ -0,0 +1 @@
+Expose the registered device ID from the `register_appservice_user` test helper.
\ No newline at end of file
diff --git a/tests/handlers/test_user_directory.py b/tests/handlers/test_user_directory.py
index 70c621b825f4..482c90ef68c5 100644
--- a/tests/handlers/test_user_directory.py
+++ b/tests/handlers/test_user_directory.py
@@ -169,7 +169,9 @@ def test_excludes_appservices_user(self) -> None:
         # Register an AS user.
         user = self.register_user("user", "pass")
         token = self.login(user, "pass")
-        as_user = self.register_appservice_user("as_user_potato", self.appservice.token)
+        as_user, _ = self.register_appservice_user(
+            "as_user_potato", self.appservice.token
+        )
 
         # Join the AS user to rooms owned by the normal user.
         public, private = self._create_rooms_and_inject_memberships(
@@ -388,7 +390,7 @@ def test_handle_local_profile_change_with_deactivated_user(self) -> None:
 
     def test_handle_local_profile_change_with_appservice_user(self) -> None:
         # create user
-        as_user_id = self.register_appservice_user(
+        as_user_id, _ = self.register_appservice_user(
             "as_user_alice", self.appservice.token
         )
 
diff --git a/tests/rest/client/test_room_batch.py b/tests/rest/client/test_room_batch.py
index 721454c1875f..e9f870403536 100644
--- a/tests/rest/client/test_room_batch.py
+++ b/tests/rest/client/test_room_batch.py
@@ -89,7 +89,7 @@ def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
         self.clock = clock
         self.storage = hs.get_storage()
 
-        self.virtual_user_id = self.register_appservice_user(
+        self.virtual_user_id, _ = self.register_appservice_user(
             "as_user_potato", self.appservice.token
         )
 
diff --git a/tests/storage/test_user_directory.py b/tests/storage/test_user_directory.py
index 7f5b28aed8c4..48f1e9d8411b 100644
--- a/tests/storage/test_user_directory.py
+++ b/tests/storage/test_user_directory.py
@@ -341,7 +341,9 @@ def test_population_excludes_appservice_user(self) -> None:
         # Register an AS user.
         user = self.register_user("user", "pass")
         token = self.login(user, "pass")
-        as_user = self.register_appservice_user("as_user_potato", self.appservice.token)
+        as_user, _ = self.register_appservice_user(
+            "as_user_potato", self.appservice.token
+        )
 
         # Join the AS user to rooms owned by the normal user.
         public, private = self._create_rooms_and_inject_memberships(
diff --git a/tests/unittest.py b/tests/unittest.py
index 14318483674e..6fc617601a40 100644
--- a/tests/unittest.py
+++ b/tests/unittest.py
@@ -620,18 +620,19 @@ def register_appservice_user(
         self,
         username: str,
         appservice_token: str,
-    ) -> str:
+    ) -> Tuple[str, str]:
         """Register an appservice user as an application service.
         Requires the client-facing registration API be registered.
 
         Args:
             username: the user to be registered by an application service.
-                Should be a full username, i.e. ""@localpart:hostname" as opposed to just "localpart"
+                Should NOT be a full username, i.e. just "localpart" as opposed to "@localpart:hostname"
             appservice_token: the acccess token for that application service.
 
         Raises: if the request to '/register' does not return 200 OK.
 
-        Returns: the MXID of the new user.
+        Returns:
+            The MXID of the new user, the device ID of the new user's first device.
         """
         channel = self.make_request(
             "POST",
@@ -643,7 +644,7 @@ def register_appservice_user(
             access_token=appservice_token,
         )
         self.assertEqual(channel.code, 200, channel.json_body)
-        return channel.json_body["user_id"]
+        return channel.json_body["user_id"], channel.json_body["device_id"]
 
     def login(
         self,