diff --git a/osx.c b/osx.c
index c3c24de..d3f7723 100644
--- a/osx.c
+++ b/osx.c
@@ -84,6 +84,7 @@ static int truncate(const char *path, __int64 size)
 }
 
 #define ftruncate(fd, size) _chsize_s(fd, size)
+#define isatty(fd) _isatty(fd)
 #define fsync(fd) _commit(fd)
 
 static int flocking(FILE *fh, int mode, __int64 ofs, __int64 len)
@@ -801,6 +802,15 @@ static int os_fadvise(lua_State *L)
     return os_result(L, ec, NULL);
 }
 
+static int os_isatty(lua_State *L)
+{
+    luaL_Stream *stream = luaL_checkudata(L, 1, LUA_FILEHANDLE);
+
+    int ec = isatty(fileno(stream->f));
+    lua_pushboolean(L, ec != 0);
+    return 1;
+}
+
 /**
  * Commit any pending write to disk.
  *
@@ -935,6 +945,7 @@ DF_OSXMOD_API int luaopen_osx(lua_State *L)
         { "setmode", os_setmode },
         { "chsize", os_chsize },
         { "fadvise", os_fadvise },
+        { "isatty", os_isatty },
         { "commit", os_commit },
         { "locking", os_locking },
         { NULL, NULL }