diff --git a/Lib/test/test_free_threading/test_bytes_object.py b/Lib/test/test_free_threading/test_bytes_object.py
@@ -1,5 +1,5 @@
 import unittest
-from threading import Thread, Barrier
+from threading import Thread, Barrier, Event
 from test.support import threading_helper
 
 threading_helper.requires_working_threading(module=True)
@@ -32,6 +32,28 @@ def work(ii):
 
             barrier.reset()
 
+    def test_racing_join_replace(self):
+        # gh-158803: join() must not use a list item that another thread
+        # replaces (and frees) concurrently.
+        lst = [bytes(10) for _ in range(100)]
+        done = Event()
+
+        def writer():
+            try:
+                for _ in range(100):
+                    for i in range(len(lst)):
+                        lst[i] = bytearray(10) if i % 2 else bytes(10)
+            finally:
+                done.set()
+
+        def reader():
+            while not done.is_set():
+                b''.join(lst)
+                b'-'.join(lst)
+                bytearray().join(lst)
+
+        threading_helper.run_concurrently([writer] + [reader] * 4)
+
 
 if __name__ == "__main__":
     unittest.main()
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-10-06-05-12-21.gh-issue-158803.iYTpRQ.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-10-06-05-12-21.gh-issue-158803.iYTpRQ.rst
@@ -0,0 +1,3 @@
+Fix a crash in :meth:`bytes.join` and :meth:`bytearray.join` in the
+:term:`free-threaded build` when another thread concurrently mutates the
+list being joined. Patch by Christian Aurich Zanettini Martins.
diff --git a/Objects/stringlib/join.h b/Objects/stringlib/join.h
@@ -5,7 +5,7 @@
 #endif
 
 Py_LOCAL_INLINE(PyObject *)
-STRINGLIB(bytes_join)(PyObject *sep, PyObject *iterable)
+STRINGLIB(bytes_join_lock_held)(PyObject *sep, PyObject *seq)
 {
     const char *sepstr = STRINGLIB_STR(sep);
     Py_ssize_t seplen = STRINGLIB_LEN(sep);
@@ -14,38 +14,29 @@ STRINGLIB(bytes_join)(PyObject *sep, PyObject *iterable)
     Py_ssize_t seqlen = 0;
     Py_ssize_t sz = 0;
     Py_ssize_t i, nbufs;
-    PyObject *seq, *item;
+    PyObject *item;
     Py_buffer *buffers = NULL;
 #define NB_STATIC_BUFFERS 10
     Py_buffer static_buffers[NB_STATIC_BUFFERS];
 #define GIL_THRESHOLD 1048576
     int drop_gil = 1;
     PyThreadState *save = NULL;
 
-    seq = PySequence_Fast(iterable, "can only join an iterable");
-    if (seq == NULL) {
-        return NULL;
-    }
-
     seqlen = PySequence_Fast_GET_SIZE(seq);
     if (seqlen == 0) {
-        Py_DECREF(seq);
         return STRINGLIB_NEW(NULL, 0);
     }
 #if !STRINGLIB_MUTABLE
     if (seqlen == 1) {
         item = PySequence_Fast_GET_ITEM(seq, 0);
         if (STRINGLIB_CHECK_EXACT(item)) {
-            Py_INCREF(item);
-            Py_DECREF(seq);
-            return item;
+            return Py_NewRef(item);
         }
     }
 #endif
     if (seqlen > NB_STATIC_BUFFERS) {
         buffers = PyMem_NEW(Py_buffer, seqlen);
         if (buffers == NULL) {
-            Py_DECREF(seq);
             PyErr_NoMemory();
             return NULL;
         }
@@ -157,13 +148,30 @@ STRINGLIB(bytes_join)(PyObject *sep, PyObject *iterable)
 error:
     res = NULL;
 done:
-    Py_DECREF(seq);
     for (i = 0; i < nbufs; i++)
         PyBuffer_Release(&buffers[i]);
     if (buffers != static_buffers)
         PyMem_Free(buffers);
     return res;
 }
 
+Py_LOCAL_INLINE(PyObject *)
+STRINGLIB(bytes_join)(PyObject *sep, PyObject *iterable)
+{
+    PyObject *seq, *res;
+
+    seq = PySequence_Fast(iterable, "can only join an iterable");
+    if (seq == NULL) {
+        return NULL;
+    }
+
+    Py_BEGIN_CRITICAL_SECTION_SEQUENCE_FAST(iterable);
+    res = STRINGLIB(bytes_join_lock_held)(sep, seq);
+    Py_END_CRITICAL_SECTION_SEQUENCE_FAST();
+
+    Py_DECREF(seq);
+    return res;
+}
+
 #undef NB_STATIC_BUFFERS
 #undef GIL_THRESHOLD