diff --git a/changelog.d/17976.doc b/changelog.d/17976.doc
new file mode 100644
index 0000000000..c45ead4cf9
--- /dev/null
+++ b/changelog.d/17976.doc
@@ -0,0 +1 @@
+Add previously-undocumented `last_seen_ts` to query user admin API.
\ No newline at end of file
diff --git a/docs/admin_api/user_admin_api.md b/docs/admin_api/user_admin_api.md
index c63b7068c5..2742d2d2cd 100644
--- a/docs/admin_api/user_admin_api.md
+++ b/docs/admin_api/user_admin_api.md
@@ -40,6 +40,7 @@ It returns a JSON body like the following:
     "erased": false,
     "shadow_banned": 0,
     "creation_ts": 1560432506,
+    "last_seen_ts": 1732919539393,
     "appservice_id": null,
     "consent_server_notice_sent": null,
     "consent_version": null,