diff --git a/Lib/test/test_free_threading/test_str.py b/Lib/test/test_free_threading/test_str.py
index 72044e979b0f48..9a1ce3620ac4b2 100644
--- a/Lib/test/test_free_threading/test_str.py
+++ b/Lib/test/test_free_threading/test_str.py
@@ -69,6 +69,22 @@ def reader_func():
         for reader in readers:
             reader.join()
 
+    def test_maketrans_dict_concurrent_modification(self):
+        for _ in range(5):
+            d = {2000: 'a'}
+
+            def work(dct):
+                for i in range(100):
+                    str.maketrans(dct)
+                    dct[2000 + i] = chr(i % 16)
+                    dct.pop(2000 + i, None)
+
+            threading_helper.run_concurrently(
+                work,
+                nthreads=5,
+                args=(d,),
+            )
+
 
 if __name__ == "__main__":
     unittest.main()
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-02-23-23-18-28.gh-issue-145142.T-XbVe.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-02-23-23-18-28.gh-issue-145142.T-XbVe.rst
new file mode 100644
index 00000000000000..5f6043cc3d9660
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2026-02-23-23-18-28.gh-issue-145142.T-XbVe.rst
@@ -0,0 +1,2 @@
+Fix a crash in the free-threaded build when the dictionary argument to
+:meth:`str.maketrans` is concurrently modified.
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index fdcbcf51cb62c2..9f90f7d27abe73 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -13155,6 +13155,7 @@ unicode_maketrans_impl(PyObject *x, PyObject *y, PyObject *z)
             goto err;
         }
         /* copy entries into the new dict, converting string keys to int keys */
+        Py_BEGIN_CRITICAL_SECTION(x);
         while (PyDict_Next(x, &i, &key, &value)) {
             if (PyUnicode_Check(key)) {
                 /* convert string keys to integer keys */
@@ -13162,27 +13163,33 @@ unicode_maketrans_impl(PyObject *x, PyObject *y, PyObject *z)
                 if (PyUnicode_GET_LENGTH(key) != 1) {
                     PyErr_SetString(PyExc_ValueError, "string keys in translate "
                                     "table must be of length 1");
-                    goto err;
+                    goto error;
                 }
                 kind = PyUnicode_KIND(key);
                 data = PyUnicode_DATA(key);
                 newkey = PyLong_FromLong(PyUnicode_READ(kind, data, 0));
                 if (!newkey)
-                    goto err;
+                    goto error;
                 res = PyDict_SetItem(new, newkey, value);
                 Py_DECREF(newkey);
                 if (res < 0)
-                    goto err;
+                    goto error;
             } else if (PyLong_Check(key)) {
                 /* just keep integer keys */
                 if (PyDict_SetItem(new, key, value) < 0)
-                    goto err;
+                    goto error;
             } else {
                 PyErr_SetString(PyExc_TypeError, "keys in translate table must "
                                 "be strings or integers");
-                goto err;
+                goto error;
             }
         }
+        goto done;
+    error:
+        Py_CLEAR(new);
+    done:
+        Py_END_CRITICAL_SECTION();
+        return new;
     }
     return new;
   err: