Sekaiichi Happy na Onna no Ko (世界一HAPPYな女の子; World's Happiest Girl) is the seventeenth major label single released by ℃-ute. The single was released on September 7, 2011 and the Single V on September 14, 2011. The single reached #6 on the Oricon charts and charted for three weeks, selling 19,830 copies, becoming their lowest selling single yet. Sekaiichi Happy na Onna no Ko was the theme song for the movie Zomvideo, starring several ℃-ute members.