diff --git a/changelog.d/20181.bugfix b/changelog.d/20181.bugfix
new file mode 100644
index 0000000000..477902b97f
--- /dev/null
+++ b/changelog.d/20181.bugfix
@@ -0,0 +1 @@
+Return the stable `M_UNKNOWN_DEVICE` error code, added in Matrix 1.17, instead of its unstable [MSC4326](https://github.com/matrix-org/matrix-spec-proposals/pull/4326)-prefixed identifier.
diff --git a/synapse/api/errors.py b/synapse/api/errors.py
index 038578dbed..75cf8add27 100644
--- a/synapse/api/errors.py
+++ b/synapse/api/errors.py
@@ -157,7 +157,7 @@ class Codes(str, Enum):
     MSC4306_NOT_IN_THREAD = "IO.ELEMENT.MSC4306.M_NOT_IN_THREAD"
 
     # Part of MSC4326
-    UNKNOWN_DEVICE = "ORG.MATRIX.MSC4326.M_UNKNOWN_DEVICE"
+    UNKNOWN_DEVICE = "M_UNKNOWN_DEVICE"
 
     USER_LIMIT_EXCEEDED = "M_USER_LIMIT_EXCEEDED"