diff --git a/Misc/NEWS.d/next/Library/2026-10-20-00-00-00.gh-issue-159739.eb8987.rst b/Misc/NEWS.d/next/Library/2026-10-20-00-00-00.gh-issue-159739.eb8987.rst
new file mode 100644
index 00000000000000..11c88411bb9125
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-10-20-00-00-00.gh-issue-159739.eb8987.rst
@@ -0,0 +1,2 @@
+Avoid undefined behavior when :func:`struct.pack_into` copies a bytearray into
+overlapping output.
diff --git a/Modules/_struct.c b/Modules/_struct.c
index 1e5d52b758f7e2..fad026ac8a4652 100644
--- a/Modules/_struct.c
+++ b/Modules/_struct.c
@@ -2409,7 +2409,7 @@ s_pack_internal(PyStructObject *soself, PyObject *const *args,
                 if (n > code->size)
                     n = code->size;
                 if (n > 0)
-                    memcpy(res, p, n);
+                    memmove(res, p, n);
             } else if (strcmp(e->format, "p") == 0) {
                 Py_ssize_t n;
                 int isstring;
@@ -2435,7 +2435,7 @@ s_pack_internal(PyStructObject *soself, PyObject *const *args,
                     n = code->size - 1;
                 }
                 if (n > 0)
-                    memcpy(res + 1, p, n);
+                    memmove(res + 1, p, n);
                 if (n > 255)
                     n = 255;
                 if (n > 0) {