Kim was discovered at Starlight Casting System October 2002 where he auditioned with his friends. Initially, he was placed in a four-member project group called, "Four Seasons". This group included members Jaejoong, Yunho, and Kangin. They never debuted as Jaejoong and Yunho joined boy band TVXQ, who debuted in December 2003. Kim and Kangin joined ten other trainees and formed the group Super Junior 05, being the first generation of the rotational boy band Super Junior.
Other activities before his debut include modeling, acting, and VJing. He made his first appearance on TV starring in the teen drama Banolim 2. Like bandmate Kibum, Kim made his first television appearance as an actor before the debut of Super Junior. He starred in the 2005 teen drama, Sharp 2, in which he acted alongside Kibum. Not long afterwards, Kim appeared in an episode of Loveholic as a chef trainee. He was a regular MC for the cable music show Show! Music Tank.