J-US (제이어스) was born in Seoul, South Korea on January 13, 1995 with the name Lee Seung-jun. After training for five years, he debuted as the Lead Dancer of the Boy Group ONF under WM Entertainment. Aside from that, he is also the leader of the OFF sub-unit.
On January 1, 2024 WM Entertainment announced that J-Us would be changing his stage name to his real name, Seungjun.
He also starred in the musical theatre Blue Helmet: A Song of Meissa on 2022.
With the exception of U, Seungjun said on November 2, 2021, that he and the other ONF members would be enlisting in December. As an active duty soldier, Seungjun enrolled on December 27. He and Wyatt were officially discharge last June 26, 2023.