diff --git a/sys/contrib/zlib/gzlib.c b/sys/contrib/zlib/gzlib.c
index b4cb5e4a48c..ea34341d5d5 100644
--- a/sys/contrib/zlib/gzlib.c
+++ b/sys/contrib/zlib/gzlib.c
@@ -7,6 +7,7 @@
 
 #include "gzguts.h"
 #include "zutil.h"
+#include <unistd.h>
 
 #if defined(_WIN32) && !defined(__BORLANDC__) && !defined(__MINGW32__)
 #  define LSEEK _lseeki64