diff --git a/Misc/NEWS b/Misc/NEWS
index a9eeeffd373..fa89eb3593a 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -32,6 +32,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #18259: Declare sethostname in socketmodule.c for AIX
+
 - Issue #18167: cgi.FieldStorage no more fails to handle multipart/form-data
   when \r\n appears at end of 65535 bytes without other newlines.
 
diff --git a/Modules/socketmodule.c b/Modules/socketmodule.c
index 9b58a11bd54..2c8a29cc929 100644
--- a/Modules/socketmodule.c
+++ b/Modules/socketmodule.c
@@ -4132,6 +4132,11 @@ socket_sethostname(PyObject *self, PyObject *args)
     Py_buffer buf;
     int res, flag = 0;
 
+#ifdef _AIX
+/* issue #18259, not declared in any useful header file */
+extern int sethostname(const char *, size_t);
+#endif
+
     if (!PyArg_ParseTuple(args, "S:sethostname", &hnobj)) {
         PyErr_Clear();
         if (!PyArg_ParseTuple(args, "O&:sethostname",