diff --git a/api/client-server/sync.yaml b/api/client-server/sync.yaml
index 4b44c20e1..f1997b617 100644
--- a/api/client-server/sync.yaml
+++ b/api/client-server/sync.yaml
@@ -227,6 +227,14 @@ paths:
                             room up to the point when the user left.
                           allOf:
                             - $ref: "definitions/timeline_batch.yaml"
+                        account_data:
+                          title: Account Data
+                          type: object
+                          description: |-
+                            The private data that this user has attached to
+                            this room.
+                          allOf:
+                          - $ref: "definitions/event_batch.yaml"
               presence:
                 title: Presence
                 type: object