diff --git a/changelog.d/12703.misc b/changelog.d/12703.misc
new file mode 100644
index 000000000..9aaa1bbaa
--- /dev/null
+++ b/changelog.d/12703.misc
@@ -0,0 +1 @@
+Convert namespace class `Codes` into a string enum.
\ No newline at end of file
diff --git a/synapse/api/errors.py b/synapse/api/errors.py
index cb3b7323d..9614be6b4 100644
--- a/synapse/api/errors.py
+++ b/synapse/api/errors.py
@@ -17,6 +17,7 @@
 
 import logging
 import typing
+from enum import Enum
 from http import HTTPStatus
 from typing import Any, Dict, List, Optional, Union
 
@@ -30,7 +31,11 @@ if typing.TYPE_CHECKING:
 logger = logging.getLogger(__name__)
 
 
-class Codes:
+class Codes(str, Enum):
+    """
+    All known error codes, as an enum of strings.
+    """
+
     UNRECOGNIZED = "M_UNRECOGNIZED"
     UNAUTHORIZED = "M_UNAUTHORIZED"
     FORBIDDEN = "M_FORBIDDEN"
@@ -265,7 +270,9 @@ class UnrecognizedRequestError(SynapseError):
     """An error indicating we don't understand the request you're trying to make"""
 
     def __init__(
-        self, msg: str = "Unrecognized request", errcode: str = Codes.UNRECOGNIZED
+        self,
+        msg: str = "Unrecognized request",
+        errcode: str = Codes.UNRECOGNIZED,
     ):
         super().__init__(400, msg, errcode)