diff --git a/changelog.d/17965.feature b/changelog.d/17965.feature
new file mode 100644
index 0000000000..e447a58986
--- /dev/null
+++ b/changelog.d/17965.feature
@@ -0,0 +1 @@
+Use stable `M_USER_LOCKED` error code for locked accounts, as per [Matrix 1.12](https://spec.matrix.org/v1.12/client-server-api/#account-locking).
\ No newline at end of file
diff --git a/synapse/api/errors.py b/synapse/api/errors.py
index e6efa7a424..71e4bb4971 100644
--- a/synapse/api/errors.py
+++ b/synapse/api/errors.py
@@ -87,8 +87,7 @@ class Codes(str, Enum):
     WEAK_PASSWORD = "M_WEAK_PASSWORD"
     INVALID_SIGNATURE = "M_INVALID_SIGNATURE"
     USER_DEACTIVATED = "M_USER_DEACTIVATED"
-    # USER_LOCKED = "M_USER_LOCKED"
-    USER_LOCKED = "ORG_MATRIX_MSC3939_USER_LOCKED"
+    USER_LOCKED = "M_USER_LOCKED"
     NOT_YET_UPLOADED = "M_NOT_YET_UPLOADED"
     CANNOT_OVERWRITE_MEDIA = "M_CANNOT_OVERWRITE_MEDIA"