diff --git a/io.c b/io.c
index 22257b65ae..83c51f4437 100644
--- a/io.c
+++ b/io.c
@@ -1247,9 +1247,11 @@ void rb_io_set_nonblock(OpenFile *fptr)
 #else
     flags = 0;
 #endif
-    flags |= O_NONBLOCK;
-    if (fcntl(fptr->fd, F_SETFL, flags) == -1) {
-        rb_sys_fail(fptr->path);
+    if ((flags & O_NONBLOCK) == 0) {
+        flags |= O_NONBLOCK;
+        if (fcntl(fptr->fd, F_SETFL, flags) == -1) {
+            rb_sys_fail(fptr->path);
+        }
     }
 }