f(x) (/ˌɛf ˈɛks/; Korean: 에프엑스; RR: Epeuekseu) is a South Korean girl group, consisting of Victoria, Amber, Luna, Krystal, and previously Sulli until her departure from the group in August 2015. Formed by SM, f(x) officially debuted in September 2009 with the release of the digital single "La Cha Ta".

Their debut studio album, Pinocchio (2011), and their two extended plays Nu Abo and Electric Shock produced three number-one singles on South Korea's Gaon Digital Chart. Their acclaimed second studio album, Pink Tape (2013), was the only K-pop album on US music channel Fuse's "41 Best Albums of 2013", the only idol album to be featured in Korean critics' definitive list of "Top 100 Classic Albums of All Time", and was named "Greatest K-pop Album of the 2010s" by Billboard. The group's third and fourth albums, Red Light (2014) and 4 Walls (2015), were commercially successful and well received by music critics. Following the release of the SM Station single "All Mine" in 2016, the group halted group promotions and the members have embarked on individual music, acting, and modelling careers. In August 2019, f(x) officially reunited as a three-piece (without Victoria) and performed at the three-night SM Town Live 2019 in Tokyo concert. As of 2025, f(x) has been removed from the artist section on the official SM Entertainment website.

Known for their eclectic, electropop-based sound, f(x) is one of the first few recognized K-pop groups internationally, becoming the first K-pop act to perform at SXSW. In 2017, Billboard ranked f(x) at number seven on their "Top 10 K-pop Girl Groups of the Past Decade" list.

f(x) is used as the notation for functions in mathematics. It is used to symbolize their various activities, based on the various talents and attractiveness of the members, like a function in which the output or results change depending on the input value of x. In addition, "f" symbolizes flower and "x" symbolizes 'X' on the female chromosome XX which, taken together, symbolizes "flower reminiscent of women".

Krystal was scouted by SM Entertainment in 2000 while on a family visit to Korea, along with her sister Jessica (who would go on to be a member of Girls' Generation). She subsequently appeared in a small role in Shinhwa's music video for the song "Wedding March" ("너의 곁에서 2"). Sulli was a child actress, making her first television appearance in 2005 as the young Princess Seonhwa of Silla in SBS's Ballad of Seodong. Luna was recruited by SM following her 2006 appearance on the reality television program Truth Game. Victoria was scouted through a Beijing dance competition in September 2007 after winning first place. In 2008, Amber was cast through S.M. Global Auditions in Los Angeles, California.^{[citation needed]}

SM introduced an upcoming girl group f(x) as a "Asia Pop Dance Group" with its goal the group focused on pop dance group. Their debut single "La Cha Ta" was released on September 1, with a showcase held the following day at Samseong Fashion Center in Gangnam. The group's broadcast debut was on September 5, on MBC's Music Core. Shortly after their debut, f(x) featured in promotions of the LG Chocolate cell phone alongside Girls' Generation. Both groups released versions of the song "Chocolate Love"; f(x)'s being an electropop version released on October 8. They also released their first physical single, "Chu~♡", on November 9. A month later, the group appeared as special guests at Girls' Generation's Into The New World concert at the Olympic Fencing Gymnasium, Seoul.

At the start of 2010, f(x) collaborated with the Chinese boy band M.I.C. to promote the LG Cyon cell phone, releasing a Chinese cover of "Lollipop". In April they appeared in their documentary reality programme, Hello f(x). On May 4, the group released their first extended play, Nu ABO. The lead single "Nu ABO" was a commercial success in Korea and become a number-one single on the Gaon Chart. In June, Amber went on hiatus due to an ankle injury, while the remaining members continued group activities without her for the remainder of the year. They promoted the single "Mr Boogie" as a follow-up single from their EP and participated in the SMTown Live '10 World Tour, as well as filming their second documentary reality show, f(x)'s Koala. Towards the end of 2010 it was announced that f(x)'s albums would be distributed under Japanese label Avex Entertainment in Japan, Taiwan, China, and Hong Kong. Amber returned to the group in early 2011.

f(x) released their first full-length studio album Pinocchio in April 2011, with the lead single "Pinocchio (Danger)". The single peaked at number one on the Gaon digital single chart, and the group went on to take eight music show wins. On June 14, Pinocchio was re-released under the title Hot Summer. The repackaged album contained the title track "Hot Summer" (a remake of a Monrose song produced by Thomas Troelsen and Remee), as well as three previously released digital singles: "LA chA TA", "Chu~♡", and "Is It Ok?". "Hot Summer" peaked at #2 on the Gaon digital single chart and was downloaded over 375,000 times in the week of release. The song went on to become one of the most successful singles of 2011 in South Korea, selling 2,909,384 copies by the end of the year. In August, the group released a song called "Garagabana" for the game Bust-a-Move and in December they released a song in English, titled "1, 2, 3", as part of the 2011 Winter SMTown – The Warmest Gift album.