A Perfect Circle is an American rock supergroup formed in 1999 by guitarist Billy Howerdel and Tool vocalist Maynard James Keenan. The original incarnation of the band also included Paz Lenchantin on bass, Troy Van Leeuwen on guitar, and Tim Alexander on drums. The latest line-up features Puscifer and Ashes Divide live bassist Matt McJunkins, former guitarist for The Smashing Pumpkins, James Iha; and session drummer Josh Freese, who is best known for his work with Nine Inch Nails, Devo, and The Vandals. Despite having a varied cast since inception, the stylistic content of the songs has remained consistent with Howerdel as composer, and Keenan penning the lyrics.

A Perfect Circle has released three albums: Mer de Noms, Thirteenth Step and Emotive. A CD-DVD set, Amotion, has also been released and contains thirteen music videos in addition to a number of song remixes created by Danny Lohner.

The band went into hiatus, in 2004. Since then band members have worked on other projects; the most notable being Keenan's work on Tool's 2006 album 10,000 Days, and Howerdel's founding of the band Ashes Divide. In a September 2008 interview, Keenan announced that he and Howerdel have been working on new material, officially ending the group's hiatus. However, the band did not start to really become active until almost two years later, in September 2010, when the band announced touring and the release of new music which may or may not be in the form of a new album.

A Perfect Circle was conceived by Billy Howerdel, a former guitar technician for Nine Inch Nails, The Smashing Pumpkins, Fishbone and Tool. Howerdel met singer Maynard James Keenan  in 1992 when Fishbone was opening for Tool, and the two became friends. Three years later, Keenan offered Howerdel, who was looking for lodging, a room in his North Hollywood  home. This provided Howerdel the opportunity to play demos of his music for Keenan. Pleased with what he heard, Keenan remarked, "I can hear myself singing. Although he originally desired a female vocalist, Howerdel agreed that Keenan would be a good fit, and A Perfect Circle was formed a short time later. They were then joined by bassist and violinist Paz Lenchantin, former Failure guitarist Troy Van Leeuwen, and the previous drummer for Primus, Tim Alexander. The band played their first show at LA's Viper Club Reception on August 15, 1999. After playing shows in L.A., the band entered the studio to begin work on their first album. Alexander was soon replaced with session drummer and member of The Vandals, Josh Freese, but Alexander's performance can still be heard on the album version of the song "The Hollow".

The band's debut album, Mer de Noms (French for "Sea of Names"), was released on May 23, 2000. The album is distinctive in that it is the highest ever debut for a rock band, selling over 188,000 copies in its first week, and appearing at number four on the Billboard 200. Howerdel had been working on some of the music, including tracks "Hollow" and "Breña", as far back as 1988. Soon after recording for the album was finished, the band began touring. Initially, they served as the opening act for Nine Inch Nails on the 2000 Fragility v2.0 tour, but they then embarked on a number of headlining tours all over the world, including the Canadian festival, Summersault. As Keenan was well known through his other band, he would often wear long wigs on his otherwise bald head when performing, to distinguish himself from his Tool persona. Mer de Noms went certified platinum by the RIAA on October 31, 2000, while the band was on tour.

The album produced three singles: "Judith", "3 Libras", and "The Hollow". As is the case with a great number of bands, A Perfect Circle added hidden touches to their songs. "Judith" is distinctive because it is named after, and about, Keenans's mother who suffered a stroke and was left paralyzed when he was 11. The song "Renholdër" is a reference to guitarist and sound engineer Danny Lohner and reads Re:D.Lohner backwards. Lohner did not know the song was about him despite his first name being sung—albeit in a distorted fashion—in the song.

In the album review by Rolling Stone, Pat Blashill wrote that Keenan "added an almost operatic angst to Howerdel's songs" and concluded that "A Perfect Circle sound like a desperate dream of what rock used to be. Maybe that's the point."Allmusic's review expressed that "there's little question that the addicting combination of Keenan's aching voice and Howerdel's accomplished songs and production skills made for one of 2000's best splashes in whatever was left of 'modern rock'."