diff --git a/changelog.d/6458.doc b/changelog.d/6458.doc
new file mode 100644
index 0000000000..3a9f831d89
--- /dev/null
+++ b/changelog.d/6458.doc
@@ -0,0 +1 @@
+Write some docs for the quarantine_media api.
diff --git a/docs/admin_api/media_admin_api.md b/docs/admin_api/media_admin_api.md
index 5e9f8e5d84..8b3666d5f5 100644
--- a/docs/admin_api/media_admin_api.md
+++ b/docs/admin_api/media_admin_api.md
@@ -21,3 +21,20 @@ It returns a JSON body like the following:
     ]
 }
 ```
+
+# Quarantine media in a room
+
+This API 'quarantines' all the media in a room.
+
+The API is:
+
+```
+POST /_synapse/admin/v1/quarantine_media/<room_id>
+
+{}
+```
+
+Quarantining media means that it is marked as inaccessible by users. It applies
+to any local media, and any locally-cached copies of remote media.
+
+The media file itself (and any thumbnails) is not deleted from the server.