diff --git a/synapse/rest/client/room.py b/synapse/rest/client/room.py
index 4fd754a06c..ab92cd3df8 100644
--- a/synapse/rest/client/room.py
+++ b/synapse/rest/client/room.py
@@ -861,7 +861,7 @@ async def encode_messages_response(
                 bundle_aggregations=get_messages_result.bundled_aggregations,
             )
         ),
-        "gaps": [
+        "org.matrix.msc3871.gaps": [
             {
                 "prev_pagination_token": await get_messages_result.start_token.copy_and_replace(
                     StreamKeyType.ROOM, gap.prev_token
diff --git a/tests/rest/client/test_rooms.py b/tests/rest/client/test_rooms.py
index 3083dd2744..1a367a47c4 100644
--- a/tests/rest/client/test_rooms.py
+++ b/tests/rest/client/test_rooms.py
@@ -2530,7 +2530,7 @@ class RoomMessageListTestCase(RoomBase):
         # Make sure the gaps are correct
         actual_gaps = [
             event_id_to_message_map.get(gap["event_id"], gap["event_id"])
-            for gap in channel.json_body["gaps"]
+            for gap in channel.json_body["org.matrix.msc3871.gaps"]
         ]
         expected_gaps = expected_messages
         # We only need to assert that gaps are in the list (the order doesn't matter)
@@ -2540,7 +2540,7 @@ class RoomMessageListTestCase(RoomBase):
             exact=True,
         )
         # Ensure that the tokens point to the correct positions
-        for gap in channel.json_body["gaps"]:
+        for gap in channel.json_body["org.matrix.msc3871.gaps"]:
             event_room_stream_token = self.get_success(
                 self.store.get_topological_token_for_event(gap["event_id"])
             )