Billie Joe Armstrong (born February 17, 1972 in Oakland, California, U.S.) is an American musician and actor. He is best known for being the lead vocalist, guitarist, and primary songwriter of the rock band Green Day, which he co-founded with Mike Dirnt in 1987. He is also a guitarist and vocalist for the punk rock band Pinhead Gunpowder, and provides lead vocals for Green Day's side projects Foxboro Hot Tubs, the Network, the Longshot and the Coverups. Armstrong has been considered by critics as one of the greatest punk rock guitarists of all time.

Armstrong developed an interest in music at a young age, and recorded his first song at the age of five. He met Dirnt while attending elementary school, and the two instantly bonded over their mutual interest in music, forming the band Sweet Children when the two were 14 years old. The band later changed its name to Green Day. Armstrong has also pursued musical projects including numerous collaborations with other musicians.

Armstrong has acted in several films and TV series, including Like Sunday, Like Rain, Ordinary World, King of the Hill, Haunted, and Drunk History. In addition to co-writing the book and lyrics for American Idiot, a jukebox musical featuring several of Green Day's songs, Armstrong portrayed the role of St. Jimmy in the show for several weeks during its run on Broadway.

Armstrong's business ventures include founding Adeline Records to help support other bands in 1997, coinciding with the release of Nimrod. Adeline signed acts such as the Frustrators, AFI, and Dillinger Four. The record company shut down two decades later in August 2017. Armstrong also co-founded Punk Bunny Coffee (formerly Oakland Coffee Works) in 2015.