diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-07-08-23-00-58.gh-issue-136421.uzieFA.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-07-08-23-00-58.gh-issue-136421.uzieFA.rst
new file mode 100644
index 00000000000000..ad2e094fcbfc83
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2025-07-08-23-00-58.gh-issue-136421.uzieFA.rst
@@ -0,0 +1,2 @@
+Fix a crash when :mod:`datetime` is being imported in multiple
+sub-interpreters at the same time.
diff --git a/Modules/_datetimemodule.c b/Modules/_datetimemodule.c
index 7a6426593d021f..90bad2de5ba4da 100644
--- a/Modules/_datetimemodule.c
+++ b/Modules/_datetimemodule.c
@@ -7336,6 +7336,8 @@ init_static_types(PyInterpreterState *interp, int reloading)
         return 0;
     }
 
+    static PyMutex mutex = {0};
+    PyMutex_Lock(&mutex);
     // `&...` is not a constant expression according to a strict reading
     // of C standards. Fill tp_base at run-time rather than statically.
     // See https://bugs.python.org/issue40777
@@ -7346,11 +7348,14 @@ init_static_types(PyInterpreterState *interp, int reloading)
      * so capi_types must have the types in the appropriate order. */
     for (size_t i = 0; i < Py_ARRAY_LENGTH(capi_types); i++) {
         PyTypeObject *type = capi_types[i];
-        if (_PyStaticType_InitForExtension(interp, type) < 0) {
+        int ret = _PyStaticType_InitForExtension(interp, type);
+        if (ret < 0) {
+            PyMutex_Unlock(&mutex);
             return -1;
         }
     }
 
+    PyMutex_Unlock(&mutex);
     return 0;
 }