f(x) (; Korean: 에프엑스; RR: Epeuekseu) was 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.

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.