Yook Sung-jae, also known mononymously as Sungjae, is a South Korean singer, songwriter, actor, model, host and entertainer. He is a member of the boy group BtoB and its sub-group, BtoB Blue. Aside from his group's activities, Yook has acted in television dramas Reply 1994, Plus Nine Boys, Who Are You: School 2015, The Village: Achiara's Secret, Guardian: The Lonely and Great God, Mystic Pop-up Bar and The Golden Spoon. He has appeared in several reality shows, including: Hitmaker, We Got Married, and Master in the House.
Yook released his debut solo album, Yook O'Clock, on March 2, 2020.