diff --git a/api/client-server/directory.yaml b/api/client-server/directory.yaml
index 78ddfa29..2c7c8386 100644
--- a/api/client-server/directory.yaml
+++ b/api/client-server/directory.yaml
@@ -148,5 +148,14 @@ paths:
               }
           schema:
             type: object
+        404:
+          description: There is no mapped room ID for this room alias.
+          examples:
+            application/json: {
+              "errcode": "M_NOT_FOUND",
+              "error": "Room alias #monkeys:example.org not found."
+            }
+          schema:
+            "$ref": "definitions/errors/error.yaml"
       tags:
         - Room directory
diff --git a/changelogs/client_server/newsfragments/2029.clarification b/changelogs/client_server/newsfragments/2029.clarification
new file mode 100644
index 00000000..95b65481
--- /dev/null
+++ b/changelogs/client_server/newsfragments/2029.clarification
@@ -0,0 +1 @@
+Add ``M_NOT_FOUND`` error definition for deleting room aliases.