Lee Jae-jin is a South Korean musician and actor, a member of rock band F.T. Island, where he performs as the bassist, 2nd vocalist. He has also contributed to FT Island's body of work with several compositions of his own. With the release of single Lovesick in 2007, the band started to gain massive renown. F.T. Island have won several awards in South Korea, including several Golden Disk Awards, Mnet Asian Music Awards and Seoul Music Awards. They have also topped popular television music program charts like Inkigayo, M! Countdown and Music Bank.
In January 2007, together with Choi Jong-hoon and Choi Min-hwan, he was put into the sub group "A3", which debuted under the label of F&C Music.
Lee Jae-jin’s older sister, Lee Chae-Won was cast in the SBS drama On Air, where she played the make-up artist for Oh Seung-ah (Kim Ha-neul).