"Bad Girl" is the debut single South Korean boy group Beast, coming from their debut mini-album Beast is the B2ST which was released in South Korea on October 14, 2009. The group released a Japanese version of the song as their second single in Japan on June 15, 2011. The Japanese single came in 4 editions: 3 CD+DVD and a regular edition. In June 7, 2011, a week before the physical release, the song peaked at no. 1 in the mobile site Recochoku - Chaku Uta's daily ringtone chart. The single peaked at no. 2 in Oricon’s Daily Chart with 21,449 copies sold on the first day and no. 3 in Oricon's Weekly Chart with 42,386 copies sold in the first week.