Kep1er (/ˈkɛplər/ ^ⓘ; Korean: 케플러) is a South Korean girl group formed by CJ ENM through the Mnet reality competition show Girls Planet 999 in 2021. The group consists of seven members: Kim Chae-hyun, Huening Bahiyyih, Choi Yu-jin, Kim Da-yeon, Seo Young-eun, Ezaki Hikaru, and Shen Xiaoting. Originally a nine-piece ensemble, former members Kang Ye-seo and Sakamoto Mashiro departed from the group in July 2024.

The group debuted on January 3, 2022, under the co-management of WakeOne and Swing Entertainment with their first extended play (EP) First Impact. Upon its release, the group received immediate commercial success, selling approximately 400,000 copies and peaking at number 1 on South Korea's Gaon Album Chart. In addition, both the EP and its lead single "Wa Da Da" charted on the Billboard Global 200 and World Digital Songs respectively. The early success subsequently hailed them as the New Wave of the Year at several awards ceremonies, including Asia Artist Awards and Seoul Music Awards.

The group's Japanese debut single, "Fly-Up", was released on September 7, 2022, under the Sony Music Entertainment Japan subsidiary Ariola Japan. Debuting at number two on Oricon Singles Chart and with over 100,000 unit sales, the single was certified gold by the Recording Industry Association of Japan (RIAJ).

Since September 30, 2024, as part of the label reorganization of Swing Entertainment, Klap Entertainment has been co-managing the group alongside WakeOne.

"Kep1er" was a name suggested by viewers of Girls Planet 999 through the website Naver. The name is a reference to the planets in the Kepler star systems named after the 16th century astronomer Johannes Kepler.

Kep1er was formed through the Mnet reality survival show Girls Planet 999, which aired from August 6 to October 22, 2021. The show brought 99 contestants from China, Japan, and South Korea to compete to debut in a multinational girl group. Out of initially 99 contestants, only the top nine would make the final lineup.

Before the show began, several group members had already been active in the entertainment industry. Choi Yu-jin made her debut in the Cube Entertainment girl group CLC on March 19, 2015, with the group's extended play (EP) First Love; she was on hiatus from CLC due to activities with Kep1er until their disbandment. In 2010, Kang Ye-seo was a member of the girl group CutieL before joining Busters from 2019 to 2020.

Kim Da-yeon participated in Produce 48 in 2018, representing CNC Entertainment, while Shen Xiaoting participated in Produce Camp 2020, representing Top Class Entertainment. Both were eliminated in the first round, placing 70th and 80th in their respective shows. Mashiro Sakamoto, a trainee at JYP Entertainment from 2016 to 2018, appeared in the first episode of the Mnet reality survival show Stray Kids as part of the female trainee team, but did not progress past the first episode. Following Produce 48, Da-yeon left CNC Entertainment and signed with Stardium Entertainment, only to leave once more after her debut plans fell through. Xiaoting was a competitive ballroom and modern dancer, having won a gold medal in a modern dance sport competition in Shanghai, as well as ranking sixth globally in a British competition.