Index: io.c
===================================================================
--- io.c	(revision 60366)
+++ io.c	(working copy)
@@ -1730,6 +1730,12 @@
     return rb_funcallv(io, id_write, 1, &str);
 }
 
+VALUE
+rb_io_writev(VALUE io, int argc, VALUE *argv)
+{
+    return rb_funcallv(io, id_write, argc, argv);
+}
+
 /*
  *  call-seq:
  *     ios << obj     -> ios
@@ -7586,6 +7592,25 @@
     return Qtrue;
 }
 
+void
+io_puts_string(VALUE out, VALUE str)
+{
+    if (RSTRING_LEN(str) == 0 || !rb_str_end_with_asciichar(str, '\n')) {
+#ifdef HAVE_WRITEV
+        VALUE args[2];
+        args[0] = str;
+        args[1] = rb_default_rs;
+        rb_io_writev(out, 2, args);
+#else
+        rb_io_write(out, str);
+        rb_io_write(out, rb_default_rs);
+#endif
+    }
+    else {
+        rb_io_write(out, str);
+    }
+}
+
 /*
  *  call-seq:
  *     ios.puts(obj, ...)    -> nil
@@ -7617,28 +7642,23 @@
 rb_io_puts(int argc, const VALUE *argv, VALUE out)
 {
     int i;
-    VALUE line;
 
     /* if no argument given, print newline. */
     if (argc == 0) {
-	rb_io_write(out, rb_default_rs);
-	return Qnil;
+        rb_io_write(out, rb_default_rs);
+        return Qnil;
     }
     for (i=0; i<argc; i++) {
-	if (RB_TYPE_P(argv[i], T_STRING)) {
-	    line = argv[i];
-	    goto string;
-	}
-	if (rb_exec_recursive(io_puts_ary, argv[i], out)) {
-	    continue;
-	}
-	line = rb_obj_as_string(argv[i]);
-      string:
-	rb_io_write(out, line);
-	if (RSTRING_LEN(line) == 0 ||
-            !rb_str_end_with_asciichar(line, '\n')) {
-	    rb_io_write(out, rb_default_rs);
-	}
+        if (RB_TYPE_P(argv[i], T_STRING)) {
+            io_puts_string(out, argv[i]);
+            continue;
+        }
+
+        if (rb_exec_recursive(io_puts_ary, argv[i], out)) {
+            continue;
+        }
+
+        io_puts_string(out, rb_obj_as_string(argv[i]));
     }
 
     return Qnil;
Index: test/mkmf/base.rb
===================================================================
--- test/mkmf/base.rb	(revision 60366)
+++ test/mkmf/base.rb	(working copy)
@@ -49,11 +49,11 @@
     def filter(&block)
       @filter = block
     end
-    def write(s)
+    def write(*args)
       if @out
-        @buffer << s
+        @buffer << args.join
       elsif @origin
-        @origin << s
+        @origin << args.join
       end
     end
   end
Index: test/ruby/test_io.rb
===================================================================
--- test/ruby/test_io.rb	(revision 60366)
+++ test/ruby/test_io.rb	(working copy)
@@ -2427,6 +2427,19 @@
     end)
   end
 
+  def test_puts_parallel
+    pipe(proc do |w|
+      threads = []
+      100.times do
+        threads << Thread.new { w.puts "hey" }
+      end
+      threads.each(&:join)
+      w.close
+    end, proc do |r|
+      assert_equal("hey\n" * 100, r.read)
+    end)
+  end
+
   def test_display
     pipe(proc do |w|
       "foo".display(w)