Yoon Byung Ok, later legally changed to Yoon Ji Sung, is a South Korean singer, dancer, actor, and former member of WANNA ONE (워너원). He graduated from National Traditional Arts High School and later obtained a bachelor's degree in Theater and Film from Myongji College and a master's degree in Performing Arts from Dongguk University.

Before debuting, he was a trainee at MMO Entertainment and served as a backup dancer of FIESTAR member Cao Lu. In mid-2017, he participated in the survival competition "Produce 101 Season 2" and managed to end up in 8th place, earning a spot in the resulting project group. He made his debut in August 2017 as WANNA ONE's leader and sub-vocalist with their first mini-album, "1X1=1 (To Be One)".