diff --git a/changelogs/server_server/newsfragments/1926.clarification b/changelogs/server_server/newsfragments/1926.clarification
new file mode 100644
index 00000000..172bea67
--- /dev/null
+++ b/changelogs/server_server/newsfragments/1926.clarification
@@ -0,0 +1 @@
+Add 403 error response to `/_matrix/federation/v1/state_ids/{roomId}`.
diff --git a/data/api/server-server/events.yaml b/data/api/server-server/events.yaml
index 9087df97..30e22830 100644
--- a/data/api/server-server/events.yaml
+++ b/data/api/server-server/events.yaml
@@ -140,6 +140,19 @@ paths:
                 required:
                   - auth_chain_ids
                   - pdu_ids
+        "403":
+          description: |-
+            The requesting host is not in the room, or is excluded from the room via `m.room.server_acl`.
+          content:
+            application/json:
+              schema:
+                $ref: ../client-server/definitions/errors/error.yaml
+              examples:
+                response:
+                  value: {
+                    "errcode": "M_FORBIDDEN",
+                    "error": "You do not have permission to access the state at the requested event."
+                  }
         "404":
           description: |-
             The given `event_id` was not found or the server doesn't know about the state at