diff --git a/Lib/test/test_types.py b/Lib/test/test_types.py
index 4595e7e5d3edc1..25135aaf2fe9fb 100644
--- a/Lib/test/test_types.py
+++ b/Lib/test/test_types.py
@@ -2167,6 +2167,21 @@ class Spam(types.SimpleNamespace):
         self.assertIs(type(spam2), Spam)
         self.assertEqual(vars(spam2), {'ham': 5, 'eggs': 9})
 
+    def test_replace_invalid_subtype(self):
+        # See https://github.com/python/cpython/issues/143636.
+        class NS(types.SimpleNamespace):
+            def __new__(cls, *args, **kwargs):
+                if created:
+                    return object()
+                return super().__new__(cls)
+
+        created = False
+        ns = NS()
+        created = True
+        err = re.escape("NS.__new__() must return an instance "
+                        "of a subclass of types.SimpleNamespace")
+        self.assertRaisesRegex(TypeError, err, copy.replace, ns)
+
     def test_fake_namespace_compare(self):
         # Issue #24257: Incorrect use of PyObject_IsInstance() caused
         # SystemError.
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-01-10-12-59-58.gh-issue-143636.dzr26e.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-01-10-12-59-58.gh-issue-143636.dzr26e.rst
new file mode 100644
index 00000000000000..4d5249ffe3a206
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2026-01-10-12-59-58.gh-issue-143636.dzr26e.rst
@@ -0,0 +1,2 @@
+Fix a crash when calling :class:`SimpleNamespace.__replace__()
+<types.SimpleNamespace>` on non-namespace instances. Patch by Bénédikt Tran.
diff --git a/Objects/namespaceobject.c b/Objects/namespaceobject.c
index 201cb8a7df8da1..42d524edd09424 100644
--- a/Objects/namespaceobject.c
+++ b/Objects/namespaceobject.c
@@ -13,6 +13,7 @@ typedef struct {
 } _PyNamespaceObject;
 
 #define _PyNamespace_CAST(op) _Py_CAST(_PyNamespaceObject*, (op))
+#define _PyNamespace_Check(op) PyObject_TypeCheck((op), &_PyNamespace_Type)
 
 
 static PyMemberDef namespace_members[] = {
@@ -234,6 +235,14 @@ namespace_replace(PyObject *self, PyObject *args, PyObject *kwargs)
     if (!result) {
         return NULL;
     }
+    if (!_PyNamespace_Check(result)) {
+        PyErr_Format(PyExc_TypeError,
+                     "%T.__new__() must return an instance of a subclass of %s",
+                     self, _PyNamespace_Type.tp_name);
+        Py_DECREF(result);
+        return NULL;
+    }
+
     if (PyDict_Update(((_PyNamespaceObject*)result)->ns_dict,
                       ((_PyNamespaceObject*)self)->ns_dict) < 0)
     {