Green Day is an American rock band formed in Rodeo, California, in 1987 by lead vocalist and guitarist Billie Joe Armstrong and bassist and backing vocalist Mike Dirnt, with drummer Tré Cool joining in 1990. In 1994, their major-label debut Dookie, released through Reprise Records, became a breakout success and eventually shipped over 10 million copies in the U.S. Alongside fellow California punk bands Bad Religion, the Offspring, Rancid, NOFX, Pennywise and Social Distortion, Green Day is credited with popularizing mainstream interest in punk rock in the U.S.

Before taking its current name in 1989, the band was named Blood Rage, then Sweet Children. They were part of the late 1980s/early 1990s Bay Area punk scene that emerged from the 924 Gilman Street club in Berkeley, California. The band's early releases were with the independent record label Lookout! Records including their first album, 39/Smooth (1990). For most of the band's career, they have been a power trio with Cool, who replaced John Kiffmeyer in 1990 before the recording of the band's second studio album, Kerplunk (1991). Though the albums Insomniac (1995), Nimrod (1997) and Warning (2000) did not match the success of Dookie, they were still successful, with Insomniac and Nimrod reaching double platinum status, while Warning achieved gold. Green Day's seventh album, a rock opera called American Idiot (2004), found popularity with a younger generation, selling six million copies in the U.S. Their next album, 21st Century Breakdown, was released in 2009 and achieved the band's best chart performance. It was followed by a trilogy of albums, ¡Uno!, ¡Dos!, and ¡Tré!, released in September, November, and December 2012, respectively. The trilogy did not perform as well as expected commercially, in comparison to their previous albums, largely due to a lack of promotion and Armstrong entering rehab. They were followed by Revolution Radio (2016), which became their third to debut at No. 1 on the Billboard 200, Father of All Motherfuckers (2020) and Saviors (2024).

In 2010, a stage adaptation of American Idiot debuted on Broadway. The musical was nominated for three Tony Awards: Best Musical, Best Scenic Design and Best Lighting Design, winning the latter two. The band was inducted into the Rock and Roll Hall of Fame in 2015, their first year of eligibility. Members of the band have collaborated on the side projects Pinhead Gunpowder, The Network, Foxboro Hot Tubs, The Longshot and The Coverups. They have also worked on solo careers.

Green Day has sold roughly 75 million records worldwide as of 2024, making them one of the best-selling music artists. The group has been nominated for 20 Grammy Awards and won five of them, with Best Alternative Album for Dookie, Best Rock Album for American Idiot and 21st Century Breakdown, Record of the Year for "Boulevard of Broken Dreams", and Best Musical Show Album for American Idiot: The Original Broadway Cast Recording.