diff --git a/python/google/protobuf/internal/thread_safe_test.py b/python/google/protobuf/internal/thread_safe_test.py
index eefe40dbf4998..3cae7887a547b 100644
--- a/python/google/protobuf/internal/thread_safe_test.py
+++ b/python/google/protobuf/internal/thread_safe_test.py
@@ -7,11 +7,13 @@
 
 """Unittest for thread safe"""
 
+import importlib
 import sys
 import threading
 import time
 import unittest
 
+from google3.fitbit.research.sensing.common.proto import evaluation_pb2
 from google.protobuf import descriptor_pb2
 from google.protobuf import descriptor_pool
 from google.protobuf import message_factory
@@ -303,6 +305,26 @@ def AccessFields(msg, barrier) -> None:
       for thread in threads:
         thread.join()
 
+  def testConcurrentGetAndRegisterMessageClassDataRace(self):
+    """Reproduces the data race in GetMessageClass/RegisterMessageClass."""
+
+    barrier = threading.Barrier(10)
+    imported = False
+    lock = threading.Lock()
+
+    def Task():
+      barrier.wait()
+      for _ in range(50):
+        evaluation_pb2.MeasurementCollection(measurements=[])
+
+        nonlocal imported
+        if not imported:
+          with lock:
+            if not imported:
+              from google3.monitoring.streamz.api import data_model_pb2
+              imported = True
+
+    self.RunThreads(10, Task)
 
 if __name__ == '__main__':
   unittest.main()
diff --git a/python/google/protobuf/pyext/descriptor.cc b/python/google/protobuf/pyext/descriptor.cc
index 5b04e3f0c96a7..c8b32f582aa26 100644
--- a/python/google/protobuf/pyext/descriptor.cc
+++ b/python/google/protobuf/pyext/descriptor.cc
@@ -541,7 +541,6 @@ static PyObject* GetConcreteClass(PyBaseDescriptor* self, void* closure) {
     return nullptr;
   }
 
-  Py_XINCREF(concrete_class);
   return concrete_class->AsPyObject();
 }
 
diff --git a/python/google/protobuf/pyext/extension_dict.cc b/python/google/protobuf/pyext/extension_dict.cc
index 76b01adbe11a6..0d8cd02cdfecf 100644
--- a/python/google/protobuf/pyext/extension_dict.cc
+++ b/python/google/protobuf/pyext/extension_dict.cc
@@ -53,12 +53,15 @@ static Py_ssize_t len(ExtensionDict* self) {
       // happened to be linked in from C++ but not imported via Python.  This is
       // for consistency with the pure Python implementation.
       if (fields[i]->file()->pool() == GetDefaultDescriptorPool()->pool &&
-          fields[i]->message_type() != nullptr &&
-          message_factory::GetMessageClass(
-              cmessage::GetFactoryForMessage(self->parent),
-              fields[i]->message_type()) == nullptr) {
-        PyErr_Clear();
-        continue;
+          fields[i]->message_type() != nullptr) {
+        CMessageClass* cls = message_factory::GetMessageClass(
+            cmessage::GetFactoryForMessage(self->parent),
+            fields[i]->message_type());
+        if (cls == nullptr) {
+          PyErr_Clear();
+          continue;
+        }
+        Py_DECREF(cls);
       }
       ++size;
     }
@@ -416,12 +419,15 @@ PyObject* IterNext(PyObject* _self) {
       // for consistency with the pure Python implementation.
       if (self->fields[index]->file()->pool() ==
               GetDefaultDescriptorPool()->pool &&
-          self->fields[index]->message_type() != nullptr &&
-          message_factory::GetMessageClass(
-              cmessage::GetFactoryForMessage(self->extension_dict->parent),
-              self->fields[index]->message_type()) == nullptr) {
-        PyErr_Clear();
-        continue;
+          self->fields[index]->message_type() != nullptr) {
+        CMessageClass* cls = message_factory::GetMessageClass(
+            cmessage::GetFactoryForMessage(self->extension_dict->parent),
+            self->fields[index]->message_type());
+        if (cls == nullptr) {
+          PyErr_Clear();
+          continue;
+        }
+        Py_DECREF(cls);
       }
 
       return PyFieldDescriptor_FromDescriptor(self->fields[index]);
diff --git a/python/google/protobuf/pyext/map_container.cc b/python/google/protobuf/pyext/map_container.cc
index e1a98809d5bb5..83e8067e9357b 100644
--- a/python/google/protobuf/pyext/map_container.cc
+++ b/python/google/protobuf/pyext/map_container.cc
@@ -306,7 +306,6 @@ PyObject* GetEntryClass(PyObject* _self) {
   CMessageClass* message_class = message_factory::GetMessageClass(
       cmessage::GetFactoryForMessage(self->parent),
       self->parent_field_descriptor->message_type());
-  Py_XINCREF(message_class);
   return reinterpret_cast<PyObject*>(message_class);
 }
 
diff --git a/python/google/protobuf/pyext/message.cc b/python/google/protobuf/pyext/message.cc
index 63f44145d08b0..01eb864039e84 100644
--- a/python/google/protobuf/pyext/message.cc
+++ b/python/google/protobuf/pyext/message.cc
@@ -2112,12 +2112,14 @@ static PyObject* ListFields(CMessage* self) {
       // happened to be linked in from C++ but not imported via Python.  This is
       // for consistency with the pure Python implementation.
       if (fields[i]->file()->pool() == GetDefaultDescriptorPool()->pool &&
-          fields[i]->message_type() != nullptr &&
-          message_factory::GetMessageClass(GetFactoryForMessage(self),
-                                           fields[i]->message_type()) ==
-              nullptr) {
-        PyErr_Clear();
-        continue;
+          fields[i]->message_type() != nullptr) {
+        CMessageClass* cls = message_factory::GetMessageClass(
+            GetFactoryForMessage(self), fields[i]->message_type());
+        if (cls == nullptr) {
+          PyErr_Clear();
+          continue;
+        }
+        Py_DECREF(cls);
       }
       ScopedPyObjectPtr extensions(GetExtensionDict(self, nullptr));
       if (extensions == nullptr) {
@@ -2687,6 +2689,7 @@ PyObject* GetFieldValue(CMessage* self,
       }
       py_container =
           NewMessageMapContainer(self, field_descriptor, value_class);
+      Py_DECREF(value_class);
     } else {
       py_container = NewScalarMapContainer(self, field_descriptor);
     }
@@ -2699,6 +2702,7 @@ PyObject* GetFieldValue(CMessage* self,
       }
       py_container = repeated_composite_container::NewContainer(
           self, field_descriptor, message_class);
+      Py_DECREF(message_class);
     } else {
       py_container =
           repeated_scalar_container::NewContainer(self, field_descriptor);
diff --git a/python/google/protobuf/pyext/message_factory.cc b/python/google/protobuf/pyext/message_factory.cc
index d8d68312d7ea0..22d114e877bd9 100644
--- a/python/google/protobuf/pyext/message_factory.cc
+++ b/python/google/protobuf/pyext/message_factory.cc
@@ -13,6 +13,7 @@
 
 #include "google/protobuf/dynamic_message.h"
 #include "google/protobuf/pyext/descriptor.h"
+#include "google/protobuf/pyext/free_threading_mutex.h"
 #include "google/protobuf/pyext/message.h"
 #include "google/protobuf/pyext/message_factory.h"
 #include "google/protobuf/pyext/scoped_pyobject_ptr.h"
@@ -31,7 +32,8 @@ namespace python {
 
 namespace message_factory {
 
-PyMessageFactory* NewMessageFactory(PyTypeObject* type, PyDescriptorPool* pool) {
+PyMessageFactory* NewMessageFactory(PyTypeObject* type,
+                                    PyDescriptorPool* pool) {
   PyMessageFactory* factory = reinterpret_cast<PyMessageFactory*>(
       PyType_GenericAlloc(type, 0));
   if (factory == nullptr) {
@@ -46,7 +48,15 @@ PyMessageFactory* NewMessageFactory(PyTypeObject* type, PyDescriptorPool* pool)
   factory->pool = pool;
   Py_INCREF(pool);
 
-  factory->classes_by_descriptor = new PyMessageFactory::ClassesByMessageMap();
+  // Explicitly construct the mutex using placement new because
+  // PyType_GenericAlloc only zero-initializes memory.
+  new (&factory->mutex) FreeThreadingMutex();
+
+  {
+    FreeThreadingLockGuard lock(factory->mutex);
+    factory->classes_by_descriptor =
+        new PyMessageFactory::ClassesByMessageMap();
+  }
 
   return factory;
 }
@@ -81,14 +91,23 @@ PyObject* New(PyTypeObject* type, PyObject* args, PyObject* kwargs) {
 static void Dealloc(PyObject* pself) {
   PyMessageFactory* self = reinterpret_cast<PyMessageFactory*>(pself);
 
-  typedef PyMessageFactory::ClassesByMessageMap::iterator iterator;
-  for (iterator it = self->classes_by_descriptor->begin();
-       it != self->classes_by_descriptor->end(); ++it) {
-    Py_CLEAR(it->second);
+  {
+    FreeThreadingLockGuard lock(self->mutex);
+    typedef PyMessageFactory::ClassesByMessageMap::iterator iterator;
+    for (iterator it = self->classes_by_descriptor->begin();
+         it != self->classes_by_descriptor->end(); ++it) {
+      Py_CLEAR(it->second);
+    }
+    delete self->classes_by_descriptor;
+    self->classes_by_descriptor = nullptr;
   }
-  delete self->classes_by_descriptor;
+
   delete self->message_factory;
   Py_CLEAR(self->pool);
+
+  // Explicitly call the destructor for the placement-new'd mutex.
+  self->mutex.~FreeThreadingMutex();
+
   PyObject_GC_UnTrack(pself);
   Py_TYPE(self)->tp_free(pself);
 }
@@ -96,6 +115,8 @@ static void Dealloc(PyObject* pself) {
 static int GcTraverse(PyObject* pself, visitproc visit, void* arg) {
   PyMessageFactory* self = reinterpret_cast<PyMessageFactory*>(pself);
   Py_VISIT(self->pool);
+
+  FreeThreadingLockGuard lock(self->mutex);
   for (const auto& desc_and_class : *self->classes_by_descriptor) {
     Py_VISIT(desc_and_class.second);
   }
@@ -106,6 +127,8 @@ static int GcClear(PyObject* pself) {
   PyMessageFactory* self = reinterpret_cast<PyMessageFactory*>(pself);
   // Here it's important to not clear self->pool, so that the C++ DescriptorPool
   // is still alive when self->message_factory is destructed.
+
+  FreeThreadingLockGuard lock(self->mutex);
   for (auto& desc_and_class : *self->classes_by_descriptor) {
     Py_CLEAR(desc_and_class.second);
   }
@@ -118,13 +141,20 @@ int RegisterMessageClass(PyMessageFactory* self,
                          const Descriptor* message_descriptor,
                          CMessageClass* message_class) {
   Py_INCREF(message_class);
-  typedef PyMessageFactory::ClassesByMessageMap::iterator iterator;
-  std::pair<iterator, bool> ret = self->classes_by_descriptor->insert(
-      std::make_pair(message_descriptor, message_class));
-  if (!ret.second) {
-    // Update case: DECREF the previous value.
-    Py_DECREF(ret.first->second);
-    ret.first->second = message_class;
+  CMessageClass* old_class = nullptr;
+  {
+    FreeThreadingLockGuard lock(self->mutex);
+    typedef PyMessageFactory::ClassesByMessageMap::iterator iterator;
+    std::pair<iterator, bool> ret = self->classes_by_descriptor->insert(
+        std::make_pair(message_descriptor, message_class));
+    if (!ret.second) {
+      // Update case: save the previous value to DECREF later.
+      old_class = ret.first->second;
+      ret.first->second = message_class;
+    }
+  }
+  if (old_class != nullptr) {
+    Py_DECREF(old_class);
   }
   return 0;
 }
@@ -134,11 +164,14 @@ CMessageClass* GetOrCreateMessageClass(PyMessageFactory* self,
   // This is the same implementation as MessageFactory.GetPrototype().
 
   // Do not create a MessageClass that already exists.
-  std::unordered_map<const Descriptor*, CMessageClass*>::iterator it =
-      self->classes_by_descriptor->find(descriptor);
-  if (it != self->classes_by_descriptor->end()) {
-    Py_INCREF(it->second);
-    return it->second;
+  {
+    FreeThreadingLockGuard lock(self->mutex);
+    std::unordered_map<const Descriptor*, CMessageClass*>::iterator it =
+        self->classes_by_descriptor->find(descriptor);
+    if (it != self->classes_by_descriptor->end()) {
+      Py_INCREF(it->second);
+      return it->second;
+    }
   }
   ScopedPyObjectPtr py_descriptor(
       PyMessageDescriptor_FromDescriptor(descriptor));
@@ -192,6 +225,7 @@ CMessageClass* GetOrCreateMessageClass(PyMessageFactory* self,
 // Retrieve the message class added to our database.
 CMessageClass* GetMessageClass(PyMessageFactory* self,
                                const Descriptor* message_descriptor) {
+  FreeThreadingLockGuard lock(self->mutex);
   typedef PyMessageFactory::ClassesByMessageMap::iterator iterator;
   iterator ret = self->classes_by_descriptor->find(message_descriptor);
   if (ret == self->classes_by_descriptor->end()) {
@@ -199,6 +233,7 @@ CMessageClass* GetMessageClass(PyMessageFactory* self,
                  std::string(message_descriptor->full_name()).c_str());
     return nullptr;
   } else {
+    Py_INCREF(ret->second);
     return ret->second;
   }
 }
diff --git a/python/google/protobuf/pyext/message_factory.h b/python/google/protobuf/pyext/message_factory.h
index 3225a130f74fc..fd1306bc0e93e 100644
--- a/python/google/protobuf/pyext/message_factory.h
+++ b/python/google/protobuf/pyext/message_factory.h
@@ -12,8 +12,10 @@
 #include <Python.h>
 
 #include <unordered_map>
+
 #include "google/protobuf/descriptor.h"
 #include "google/protobuf/pyext/descriptor_pool.h"
+#include "google/protobuf/pyext/free_threading_mutex.h"
 
 namespace google {
 namespace protobuf {
@@ -45,7 +47,10 @@ struct PyMessageFactory {
   // Python references to classes are owned by this PyDescriptorPool.
   typedef std::unordered_map<const Descriptor*, CMessageClass*>
       ClassesByMessageMap;
-  ClassesByMessageMap* classes_by_descriptor;
+  ClassesByMessageMap* classes_by_descriptor ABSL_GUARDED_BY(mutex);
+
+  // Mutex to protect classes_by_descriptor in free-threaded builds.
+  FreeThreadingMutex mutex;
 };
 
 extern PyTypeObject PyMessageFactory_Type;
@@ -61,7 +66,7 @@ int RegisterMessageClass(PyMessageFactory* self,
                          const Descriptor* message_descriptor,
                          CMessageClass* message_class);
 // Retrieves the Python class registered with the given message descriptor, or
-// fail with a TypeError. Returns a *borrowed* reference.
+// fail with a TypeError. Returns a *new* reference.
 CMessageClass* GetMessageClass(PyMessageFactory* self,
                                const Descriptor* message_descriptor);
 // Retrieves the Python class registered with the given message descriptor.