Kim made his debut as a member of SS501 on 23 June 2005, with the group's first EP, Warning, released by DSP Media. Their second EP, Snow Prince was released in late 2005, five months after their debut. The group quickly gained popularity, winning multiple new artist awards in 2005 and 2006. The following year, in 2007, SS501 debuted in Japan with the single "Kokoro," which reached number five on the Oricon charts. In January 2008, the group received a Newcomer Award at the Japan Gold Disc Award ceremony, making them one of only a few South Korean artists to win this award.