diff --git a/Lib/test/test_thread.py b/Lib/test/test_thread.py
index ac924728febc991..3776371ee21b63f 100644
--- a/Lib/test/test_thread.py
+++ b/Lib/test/test_thread.py
@@ -345,6 +345,48 @@ def func():
             handle = thread.start_joinable_thread(func, handle=None)
             handle.join()
 
+class StartNewThreadKwargsRace(unittest.TestCase):
+
+    @unittest.skipUnless(support.Py_GIL_DISABLED, "GIL must be disabled")
+    def test_dict_growsup_when_thread_start(self):
+        # See gh-149816 - (62) Concurrent kwargs growth causes heap overwrite
+        # This test is meant to be run under a free-threaded build, where the GIL is
+        # disabled and concurrent mutations of the same dict can cause heap
+        # corruption.
+        results = []
+        def mutator(shared, stop, prefix, burst):
+            i = 0
+            while not stop.locked():
+                for _ in range(burst):
+                    shared[f"{prefix}_{i}"] = i
+                    i += 1
+                time.sleep(0)
+            results.append(prefix)
+
+        def nop(i, **kwargs):
+            pass
+
+        DELAY = 1.0
+        stop = thread.lock()
+        shared = {f"base_{i}": i for i in range(20000)}
+        n = 4
+        for i in range(n):
+            args=(shared, stop, f"dynamic_{i}", 1000)
+            thread.start_new_thread(mutator, args)
+
+        snt = 32
+        for i in range(snt):
+            try:
+                thread.start_new_thread(nop, (i,), shared)
+            except RuntimeError:
+                break
+
+        stop.acquire()
+        # wait for all mutator threads stop.
+        wait_t = time.monotonic()
+        while len(results) < n and time.monotonic() - wait_t < DELAY:
+            time.sleep(0.01)
+
 
 class Barrier:
     def __init__(self, num_threads):
diff --git a/Misc/NEWS.d/next/Library/2026-05-20-19-10-09.gh-issue-149816.g_ycIN.rst b/Misc/NEWS.d/next/Library/2026-05-20-19-10-09.gh-issue-149816.g_ycIN.rst
new file mode 100644
index 000000000000000..50dc74c8beeaca2
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-05-20-19-10-09.gh-issue-149816.g_ycIN.rst
@@ -0,0 +1 @@
+Fix a race condition on ``kwargs`` in ``PyStack_UnpackDict`` by duplicating the ``kwargs`` argument in the ``thread.new_start_thread`` function.
diff --git a/Modules/_threadmodule.c b/Modules/_threadmodule.c
index 135b53111014d1c..c70c454e171868e 100644
--- a/Modules/_threadmodule.c
+++ b/Modules/_threadmodule.c
@@ -385,6 +385,21 @@ thread_run(void *boot_raw)
     PyEval_AcquireThread(tstate);
     _Py_atomic_add_ssize(&tstate->interp->threads.count, 1);
 
+#ifdef Py_GIL_DISABLED
+    // See gh-149816 - (62) Concurrent kwargs growth causes heap overwrite
+    // So duplicate boot->kwargs to ensure that it won't be mutated concurrently
+    // by the caller.
+    if (boot->kwargs != NULL) {
+        PyObject *n_kwargs = PyDict_Copy(boot->kwargs);
+        if (n_kwargs == NULL) {
+            thread_bootstate_free(boot, 1);
+            goto exit;
+        }
+        Py_DECREF(boot->kwargs); // I am not pretty sure about this.
+        boot->kwargs = n_kwargs;
+    }
+#endif /* Py_GIL_DISABLED */
+
     PyObject *res = PyObject_Call(boot->func, boot->args, boot->kwargs);
     if (res == NULL) {
         if (PyErr_ExceptionMatches(PyExc_SystemExit))