diff --git a/api/client-server/v1/sync.yaml b/api/client-server/v1/sync.yaml
index d07e9399..8517ae4d 100644
--- a/api/client-server/v1/sync.yaml
+++ b/api/client-server/v1/sync.yaml
@@ -245,7 +245,12 @@ paths:
                                   "user_id": "@alice:localhost"
                               }
                           ],
-                          "visibility": "private"
+                          "visibility": "private",
+                          "private_user_data": [{
+                            "type": "m.tag",
+                            "content": {"tags": ["work"]},
+                            "room_id": "!TmaZBKYIFrIPVGoUYp:localhost"
+                          }]
                       }
                   ]
               }
@@ -332,6 +337,16 @@ paths:
                       description: |-
                         Whether this room is visible to the ``/publicRooms`` API
                         or not."
+                    private_user_data:
+                      type: array
+                      description: |-
+                        The private data that this user has attached to
+                        this room.
+                      items:
+                        title: Event
+                        type: object
+                        allOf:
+                          - "$ref": "core-event-schema/event.json"
                   required: ["room_id", "membership"]
             required: ["end", "rooms", "presence"]
         404: