diff --git a/Lib/test/test_mmap.py b/Lib/test/test_mmap.py
index 59be17455a4e4e..e7b6ff39fdb9a1 100644
--- a/Lib/test/test_mmap.py
+++ b/Lib/test/test_mmap.py
@@ -795,6 +795,32 @@ def test_weakref(self):
         gc_collect()
         self.assertIs(wr(), None)
 
+    def test_write_overlapping_memoryview(self):
+        data = bytes(range(256)) * 16
+        expected = data[:1] + data[:-1]
+        with mmap.mmap(-1, len(data)) as mm, memoryview(mm) as view:
+            mm[:] = data
+            mm.seek(1)
+            self.assertEqual(mm.write(view[:-1]), len(data) - 1)
+            self.assertEqual(mm[:], expected)
+            mm[:] = data
+            mm[1:] = view[:-1]
+            self.assertEqual(mm[:], expected)
+
+    def test_extended_slice_overlapping_memoryview(self):
+        data = b'abcdefghijkl'
+        for step in (-3, -2, -1, 2, 3):
+            target = slice(None, None, step)
+            size = len(data[target])
+            for offset in (0, len(data) - size):
+                with self.subTest(step=step, offset=offset):
+                    expected = bytearray(data)
+                    expected[target] = data[offset:offset + size]
+                    with mmap.mmap(-1, len(data)) as mm, memoryview(mm) as view:
+                        mm[:] = data
+                        mm[target] = view[offset:offset + size]
+                        self.assertEqual(mm[:], expected)
+
     def test_write_returning_the_number_of_bytes_written(self):
         mm = mmap.mmap(-1, 16)
         self.assertEqual(mm.write(b""), 0)
diff --git a/Misc/NEWS.d/next/Library/2026-10-21-21-26-10.gh-issue-159739.yuOQX5.rst b/Misc/NEWS.d/next/Library/2026-10-21-21-26-10.gh-issue-159739.yuOQX5.rst
new file mode 100644
index 00000000000000..71e75d277cc554
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-10-21-21-26-10.gh-issue-159739.yuOQX5.rst
@@ -0,0 +1 @@
+Fix overlapping writes and slice assignments to :class:`mmap.mmap` objects.
diff --git a/Modules/mmapmodule.c b/Modules/mmapmodule.c
index ce5c59cb021270..7c9fae2ba96042 100644
--- a/Modules/mmapmodule.c
+++ b/Modules/mmapmodule.c
@@ -752,7 +752,7 @@ mmap_mmap_write_impl(mmap_object *self, Py_buffer *data)
 
     CHECK_VALID(NULL);
     PyObject *result;
-    if (safe_memcpy(self->data + self->pos, data->buf, data->len) < 0) {
+    if (safe_memmove(self->data + self->pos, data->buf, data->len) < 0) {
         result = NULL;
     }
     else {
@@ -1722,16 +1722,27 @@ mmap_ass_subscript_impl(mmap_object *self, PyObject *item, PyObject *value)
         if (slicelen == 0) {
         }
         else if (step == 1) {
-            if (safe_memcpy(self->data + start, vbuf.buf, slicelen) < 0) {
+            if (safe_memmove(self->data + start, vbuf.buf, slicelen) < 0) {
                 result = -1;
             }
         }
         else {
-            if (safe_copy_from_slice(self->data, (char *)vbuf.buf, start, step,
-                                     slicelen) < 0)
-            {
+            /* The source may overlap the destination, so copy it before
+               assigning to the extended slice. */
+            char *source = PyMem_Malloc(slicelen);
+            if (source == NULL) {
+                PyErr_NoMemory();
                 result = -1;
             }
+            else {
+                if (safe_memcpy(source, vbuf.buf, slicelen) < 0 ||
+                    safe_copy_from_slice(self->data, source, start, step,
+                                         slicelen) < 0)
+                {
+                    result = -1;
+                }
+                PyMem_Free(source);
+            }
         }
         PyBuffer_Release(&vbuf);
         return result;