Metallica is an American heavy metal band. It was formed in Los Angeles in 1981 by vocalist and guitarist James Hetfield and drummer Lars Ulrich, and has been based in San Francisco for most of its career. The band's fast tempos, instrumentals and aggressive musicianship made them one of the founding "big four" bands of thrash metal, alongside Megadeth, Anthrax and Slayer. Metallica's current lineup comprises founding members and primary songwriters Hetfield and Ulrich, longtime lead guitarist Kirk Hammett and bassist Robert Trujillo. Guitarist Dave Mustaine, who formed Megadeth after being fired from Metallica, and bassists Ron McGovney, Cliff Burton and Jason Newsted are former members of the band.

Metallica first found commercial success with the release of its third album, Master of Puppets (1986), which is cited as one of the heaviest metal albums and the band's best work. The band's next album, ...And Justice for All (1988), gave Metallica its first Grammy Award nomination. Its fifth album, Metallica (1991), was a turning point for the band that saw them transition from their thrash roots; it appealed to a more mainstream audience, achieving substantial commercial success and selling more than 16 million copies in the United States to date, making it the best-selling album of the SoundScan era. After experimenting with different genres and directions in subsequent releases, Metallica returned to its thrash metal roots with its ninth album, Death Magnetic (2008), which drew similar praise to that of the band's earlier albums. The band's eleventh and most recent album, 72 Seasons, was released in 2023.

In 2000, Metallica led the case against the peer-to-peer file sharing service Napster, in which the band and several other artists filed lawsuits against the service for sharing their copyright-protected material without consent, eventually reaching a settlement. Metallica was the subject of the acclaimed 2004 documentary film Metallica: Some Kind of Monster, which documented the troubled production of the band's eighth album, St. Anger (2003), and the internal struggles within the band at the time. In 2009, Metallica was inducted into the Rock and Roll Hall of Fame. The band co-wrote the screenplay for and starred alongside Dane DeHaan in the 2013 concert film Metallica: Through the Never, in which the band performed live against a fictional thriller storyline.

Metallica has released eleven studio albums, four live albums (including two performances with the San Francisco Symphony), twelve video albums, one cover album, two extended plays, 37 singles and 39 music videos. The band has won ten Grammy Awards from 26 nominations and had six consecutive studio albums – from Metallica through Hardwired... to Self-Destruct (2016) – debut at number one on the Billboard 200. Metallica ranks as one of the most commercially successful bands of all time, having sold more than 125 million albums worldwide as of 2018. Metallica has been listed as one of the greatest artists of all time by magazines such as Rolling Stone, which ranked the band in 61st place on its list of 100 greatest artists of all time. As of 2017, Metallica is the third-best-selling music artist since Nielsen SoundScan began tracking sales in 1991, selling 58 million albums in the United States.