Lee Seung Jun, formerly known as J-US and now known mononymously as Seungjun, is a South Korean singer, choreographer, and member of ONF (온앤오프; short for ON N' OFF). He graduated from Deoksu High School and was enrolled in the Musical Studies program at the International University of the Arts but dropped out. He later earned a bachelor's degree in Japanese from Hanyang Cyber University.

Before debuting, he was a trainee for 5 years and was introduced as part of WM Boys with their first appearance as B1A4's backup dancers at the 2016 Dream Concert in Seoul. He was featured as a dancer in the Korean version of the web drama "Loss:Time:Life" and gave a performance at IDOLCON just a few months before debuting.

He made his official debut in August 2017 as the group's OFF Team leader, lead dancer and sub-vocalist with their 1st EP, "ON/OFF." In 2018, the group signed a contract with Victor Entertainment (ビクターエンタテインメント) and made their Japanese debut and held a concert in Tokyo. He had his acting debut with a role in the web drama "Can I Step In?."

In 2021, all members (except U) decided to enlist for their mandatory military service around the same time, so he enlisted on December 27 and was discharged on June 26, 2023.

The group was represented by WM Entertainment (WM엔터테인먼트).

On March 5, 2026, he signed an exclusive contract with KI Entertainment.

(Source: MyDramaList) Edit Biography