NCT 127 (Korean: 엔시티 127; RR: ensiti il-i-chil) is the first fixed and second overall sub-unit of the South Korean boy band NCT, formed and managed by SM Entertainment. They debuted on July 7, 2016 with the single "Firetruck" and an original lineup of seven members: Taeil, Taeyong, Yuta, Jaehyun, Winwin, Mark, and Haechan. Their debut extended play (EP) NCT #127 (2016) was a commercial success upon release, winning them the Best New Group title at several South Korean music award shows. Members Doyoung and Johnny joined the unit in December 2016, ahead of their second EP Limitless (2017); Jungwoo, the final member, was introduced in September 2018 for the group's first studio album Regular-Irregular (2018).

Since their debut, NCT 127 has released three studio albums, two repackaged albums, and four extended plays in South Korea. They broke into the mainstream in 2017 with "Cherry Bomb", now recognized as one of their signature songs, and received greater international attention following the release of their first studio album and its lead single "Regular" in 2018.

In April 2019, the unit's increasing global popularity prompted their label SM Entertainment to sign marketing and distribution deals with Capitol Music Group and Caroline Distribution, respectively. NCT 127 subsequently released the EP We Are Superhuman that year and their second Korean studio album, Neo Zone, in 2020, the latter becoming their first top-five entry on the US Billboard 200 and their first release to sell over one million copies. In 2021, NCT 127 released their record-breaking third Korean studio album, Sticker, and its repackage album Favorite, which sold a combined 3.58 million copies and became the best-selling album of all-time by an act under SM Entertainment. The album also debuted at number three on the US Billboard 200, becoming the highest-ranking K-pop album on the chart in 2021. NCT 127 have also ventured into the Japanese music scene with a Japanese version of their second single "Limitless", an EP, and a studio album.