diff --git a/changelog.d/20154.bugfix b/changelog.d/20154.bugfix
new file mode 100644
index 00000000000..50de48799f7
--- /dev/null
+++ b/changelog.d/20154.bugfix
@@ -0,0 +1 @@
+Return `allowed_room_ids` in the client [`GET /_matrix/client/v1/rooms/{roomId}/hierarchy`](https://spec.matrix.org/v1.19/client-server-api/#get_matrixclientv1roomsroomidhierarchy) response, as required since Matrix 1.15.
diff --git a/synapse/handlers/room_summary.py b/synapse/handlers/room_summary.py
index 4afd449292c..9dcdc8e2ad4 100644
--- a/synapse/handlers/room_summary.py
+++ b/synapse/handlers/room_summary.py
@@ -368,7 +368,7 @@ async def _get_room_hierarchy(
             # inaccessible to the requesting user.
             if room_entry:
                 # Add the room (including the stripped m.space.child events).
-                rooms_result.append(room_entry.as_json(for_client=True))
+                rooms_result.append(room_entry.as_json())
 
                 # If this room is not at the max-depth, check if there are any
                 # children to process.
@@ -872,7 +872,8 @@ async def get_room_summary(
         remote_room_hosts: list[str] | None = None,
     ) -> JsonDict:
         """
-        Implementation of the room summary C-S API from MSC3266
+        Implementation of the room summary C-S API, see
+        https://spec.matrix.org/v1.19/client-server-api/#get_matrixclientv1room_summaryroomidoralias
 
         Args:
             requester:  user id of the user making this request, will be None
@@ -980,25 +981,14 @@ class _RoomEntry:
     # This may not include all children.
     children_state_events: Sequence[JsonDict] = ()
 
-    def as_json(self, for_client: bool = False) -> JsonDict:
+    def as_json(self) -> JsonDict:
         """
         Returns a JSON dictionary suitable for the room hierarchy endpoint.
 
         It returns the room summary including the stripped m.space.child events
         as a sub-key.
-
-        Args:
-            for_client: If true, any server-server only fields are stripped from
-                the result.
-
         """
         result = dict(self.room)
-
-        # Before returning to the client, remove the allowed_room_ids key, if it
-        # exists.
-        if for_client:
-            result.pop("allowed_room_ids", False)
-
         result["children_state"] = self.children_state_events
         return result
 
diff --git a/tests/handlers/test_room_summary.py b/tests/handlers/test_room_summary.py
index 0f8de6e7b92..bb6284b2fc7 100644
--- a/tests/handlers/test_room_summary.py
+++ b/tests/handlers/test_room_summary.py
@@ -31,6 +31,7 @@
     JoinRules,
     Membership,
     RestrictedJoinRuleTypes,
+    RoomEncryptionAlgorithms,
     RoomTypes,
 )
 from synapse.api.errors import AuthError, NotFoundError, SynapseError
@@ -184,9 +185,6 @@ def _assert_hierarchy(
         result_room_ids = []
         result_children_ids = []
         for result_room in result["rooms"]:
-            # Ensure federation results are not leaking over the client-server API.
-            self.assertNotIn("allowed_room_ids", result_room)
-
             result_room_ids.append(result_room["room_id"])
             result_children_ids.append(
                 [
@@ -489,6 +487,44 @@ def test_filtering(self) -> None:
         )
         self._assert_hierarchy(result, expected)
 
+    def test_summary_fields(self) -> None:
+        """
+        The room entries returned to clients include the `room_version`,
+        `encryption` and `allowed_room_ids` fields, added to the client
+        `/hierarchy` API in Matrix 1.15.
+
+        See https://spec.matrix.org/v1.19/client-server-api/#get_matrixclientv1roomsroomidhierarchy
+        """
+        restricted_room = self._create_room_with_join_rule(
+            JoinRules.RESTRICTED,
+            room_version=RoomVersions.V8.identifier,
+            allow=[
+                {
+                    "type": RestrictedJoinRuleTypes.ROOM_MEMBERSHIP,
+                    "room_id": self.space,
+                    "via": [self.hs.hostname],
+                }
+            ],
+        )
+        self.helper.send_state(
+            restricted_room,
+            event_type=EventTypes.RoomEncryption,
+            body={"algorithm": RoomEncryptionAlgorithms.DEFAULT},
+            tok=self.token,
+        )
+
+        result = self.get_success(
+            self.handler.get_room_hierarchy(create_requester(self.user), self.space)
+        )
+        room_entries = {entry["room_id"]: entry for entry in result["rooms"]}
+        entry = room_entries[restricted_room]
+        self.assertEqual(entry["room_version"], RoomVersions.V8.identifier)
+        self.assertEqual(entry["encryption"], RoomEncryptionAlgorithms.DEFAULT)
+        self.assertEqual(entry["allowed_room_ids"], [self.space])
+
+        # Rooms without restricted join rules should not have the field at all.
+        self.assertNotIn("allowed_room_ids", room_entries[self.room])
+
     def test_complex_space(self) -> None:
         """
         Create a "complex" space to see how it handles things like loops and subspaces.
@@ -960,6 +996,13 @@ async def summarize_remote_room_hierarchy(
             )
         self._assert_hierarchy(result, expected)
 
+        # `allowed_room_ids` returned over federation should be passed through
+        # to the client.
+        room_entries = {entry["room_id"]: entry for entry in result["rooms"]}
+        self.assertEqual(
+            room_entries[restricted_accessible_room]["allowed_room_ids"], [self.room]
+        )
+
     def test_fed_invited(self) -> None:
         """
         A room which the user was invited to should be included in the response.