fromis 9 (Hangul: 프로미스나인; initialism for From Idol School; stylized as fromis_9) is a nine-member South Korean girl group formed by Off The Record Entertainment through the 2017 reality show Idol School on Mnet. The group's name, pronounced as "promise" in Korean, also means "to keep their promise to be the best girl group". The group composed of: Lee Saerom (이새롬), Song Hayoung (송하영), Jang Gyuri (장규리), Park Jiwon (박지원), Roh Jisun (노지선), Lee Seoyeon (이서연), Lee Chaeyoung (이채영), Lee Nagyung (이나경) and Baek Jiheon (백지헌).

On November 29, fromis_9 performed their predebut single titled "Glass Shoes" at the 2017 Mnet Asian Music Awards in Japan. The song was released as a digital single the following day. Their debut album was then announced to be released in January. On January 24, 2018, fromis_9 released their first mini-album To. Heart and title track "To Heart". The album peaked at #4 on the Gaon Album Chart.

On May 10, it was confirmed that member Gyuri would be participating in Mnet's Produce 48 as a contestant. Fromis_9 continued their promotions as a eight-member group, returning with their second album To. Day and title track "두근두근 (DKDK)" on June 5. The album peaked at #4 on the Gaon Album Chart.