diff --git a/Lib/test/test_free_threading/test_io.py b/Lib/test/test_free_threading/test_io.py
index 8a0ad30c4bc770b..f59a3a2e9720436 100644
--- a/Lib/test/test_free_threading/test_io.py
+++ b/Lib/test/test_free_threading/test_io.py
@@ -117,6 +117,33 @@ def sizeof(barrier, b, *ignore):
 class CBytesIOTest(ThreadSafetyMixin, TestCase):
     ioclass = io.BytesIO
 
+    @threading_helper.requires_working_threading()
+    @threading_helper.reap_threads
+    def test_concurrent_whole_buffer_read_and_resize(self):
+        shared = self.ioclass(b"x" * 64)
+        writers = 2
+        readers = 8
+        loops = 2000
+        barrier = threading.Barrier(writers + readers)
+
+        def writer():
+            barrier.wait()
+            for i in range(loops):
+                shared.seek(0)
+                shared.write(b"a" * (64 + (i & 63)))
+
+        def reader():
+            barrier.wait()
+            for _ in range(loops):
+                shared.seek(0)
+                shared.read()
+                shared.getvalue()
+
+        threads = [threading.Thread(target=writer) for _ in range(writers)]
+        threads += [threading.Thread(target=reader) for _ in range(readers)]
+        with threading_helper.start_threads(threads):
+            pass
+
 class PyBytesIOTest(ThreadSafetyMixin, TestCase):
      ioclass = pyio.BytesIO
 
diff --git a/Misc/NEWS.d/next/Library/2026-06-18-12-48-03.gh-issue-151640.R4c3Fx.rst b/Misc/NEWS.d/next/Library/2026-06-18-12-48-03.gh-issue-151640.R4c3Fx.rst
new file mode 100644
index 000000000000000..7fb6e9aee6cfd5e
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-06-18-12-48-03.gh-issue-151640.R4c3Fx.rst
@@ -0,0 +1,3 @@
+Fix a data race in :class:`io.BytesIO` in free-threaded builds when
+whole-buffer reads or :meth:`~io.BytesIO.getvalue` share the internal buffer
+with concurrent writes.
diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index 8cdcbd0d89c718e..908552c4f14c6f3 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -371,6 +371,10 @@ _io_BytesIO_getvalue_impl(bytesio *self)
 /*[clinic end generated code: output=b3f6a3233c8fd628 input=c91bff398df0c352]*/
 {
     CHECK_CLOSED(self);
+#ifdef Py_GIL_DISABLED
+    return PyBytes_FromStringAndSize(PyBytes_AS_STRING(self->buf),
+                                     self->string_size);
+#else
     if (self->string_size <= 1 || FT_ATOMIC_LOAD_SSIZE_RELAXED(self->exports) > 0)
         return PyBytes_FromStringAndSize(PyBytes_AS_STRING(self->buf),
                                          self->string_size);
@@ -386,6 +390,7 @@ _io_BytesIO_getvalue_impl(bytesio *self)
         }
     }
     return Py_NewRef(self->buf);
+#endif
 }
 
 /*[clinic input]
@@ -429,12 +434,14 @@ read_bytes_lock_held(bytesio *self, Py_ssize_t size)
 
     assert(self->buf != NULL);
     assert(size <= self->string_size);
+#ifndef Py_GIL_DISABLED
     if (size > 1 &&
         self->pos == 0 && size == PyBytes_GET_SIZE(self->buf) &&
         FT_ATOMIC_LOAD_SSIZE_RELAXED(self->exports) == 0) {
         self->pos += size;
         return Py_NewRef(self->buf);
     }
+#endif
 
     /* gh-141311: Avoid undefined behavior when self->pos (limit PY_SSIZE_T_MAX)
        is beyond the size of self->buf. Assert above validates size is always in