diff --git a/proto/marshal/marshal.py b/proto/marshal/marshal.py
index d278421a..37ae137e 100644
--- a/proto/marshal/marshal.py
+++ b/proto/marshal/marshal.py
@@ -14,6 +14,7 @@
 
 import abc
 import enum
+import threading
 
 from google.protobuf import message
 from google.protobuf import duration_pb2
@@ -256,6 +257,7 @@ class Marshal(BaseMarshal):
     """
 
     _instances = {}
+    _instances_lock = threading.Lock()
 
     def __new__(cls, *, name: str):
         """Create a marshal instance.
@@ -267,7 +269,13 @@ def __new__(cls, *, name: str):
         """
         klass = cls._instances.get(name)
         if klass is None:
-            klass = cls._instances[name] = super().__new__(cls)
+            with cls._instances_lock:
+                klass = cls._instances.get(name)
+                if klass is None:
+                    klass = super().__new__(cls)
+                    instances_copy = cls._instances.copy()
+                    instances_copy[name] = klass
+                    cls._instances = instances_copy
 
         return klass