NCT 127 is a sub-unit of NCT (2), formed and managed by S.M. Entertainment. The unit is based in Seoul, with the number 127 representing the longitude coordinates of Seoul. They made their stage debut on July 7, 2016 on M! Countdown, performing "Fire Truck" and "Once Again" from their album. As of April 2026, the group consists of seven members: Taeyong (leader), Johnny, Yuta, Doyoung, Jaehyun, Jungwoo, and Haechan (formerly known as Donghyuck).

The group's original line-up was: Taeil, Taeyong, Yuta, Jaehyun, Winwin, Mark, and Haechan. Doyoung and Johnny joined the unit in December 2016. Jungwoo, the final member, was introduced in September 2018 ahead of their first studio album Regular-Irregular. In 2018, Winwin moved on to promote with another NCT sub-unit, WayV. In August 2024, S.M. Entertainment announced Taeil's removal from NCT following sexual offense allegations. Mark announced his departure from NCT and SM Entertainment in April 2026.

Part of their rap line (Taeyong and Mark) was selected to become part of the SM super group SuperM in 2019.