NCT U is the first unit of the South Korean boy group NCT. NCT U is a rotational unit without a final lineup. The members will change in every release. The unit debuted in April 2016, with the double digital singles "The 7th Sense" and "WITHOUT YOU", with its first lineup: Taeyong, Doyoung, Ten, Jaehyun and Mark.

In January 2018, Doyoung and Jaehyun released a digital single "Timeless" under NCT U, as a part of the SM STATION series.

In the same month, SM Entertainment announced a "large-scale project" for the year, titled NCT 2018. The whole group will release an album in March as 18 members, which will include songs from all the debuted units.

In September 2020, NCT announced a reunion for a second group project, NCT 2020, in October 2020. The first part of their second album, NCT 2020 Resonance Pt. 1, was released on October 12. The record featured all four existing units and two new members, Shotaro and Sungchan. NCT released NCT 2020 Resonance Pt. 2 on November.