diff --git a/changelog.d/5686.feature b/changelog.d/5686.feature
new file mode 100644
index 000000000..367aa1eca
--- /dev/null
+++ b/changelog.d/5686.feature
@@ -0,0 +1 @@
+Use `M_USER_DEACTIVATED` instead of `M_UNKNOWN` for errcode when a deactivated user attempts to login.
diff --git a/synapse/api/errors.py b/synapse/api/errors.py
index ad3e26204..cf1ebf1af 100644
--- a/synapse/api/errors.py
+++ b/synapse/api/errors.py
@@ -61,6 +61,7 @@ class Codes(object):
     INCOMPATIBLE_ROOM_VERSION = "M_INCOMPATIBLE_ROOM_VERSION"
     WRONG_ROOM_KEYS_VERSION = "M_WRONG_ROOM_KEYS_VERSION"
     EXPIRED_ACCOUNT = "ORG_MATRIX_EXPIRED_ACCOUNT"
+    USER_DEACTIVATED = "M_USER_DEACTIVATED"
 
 
 class CodeMessageException(RuntimeError):
@@ -151,7 +152,7 @@ class UserDeactivatedError(SynapseError):
             msg (str): The human-readable error message
         """
         super(UserDeactivatedError, self).__init__(
-            code=http_client.FORBIDDEN, msg=msg, errcode=Codes.UNKNOWN
+            code=http_client.FORBIDDEN, msg=msg, errcode=Codes.USER_DEACTIVATED
         )