Timothy Ross Armstrong is an American musician, singer, songwriter, producer, and actor. Known for his distinctive hoarse and soulful voice, he is the singer/guitarist for the punk rock band Rancid and hip hop/punk rock supergroup Transplants. Prior to forming Rancid, Armstrong was in the ska punk band Operation Ivy.

In 1997, along with Brett Gurewitz of the band Bad Religion and owner of Epitaph Records, Armstrong founded Hellcat Records. In 2012, through his website, Armstrong started releasing music that influenced him, along with stripped-down cover songs of his own under the name Tim Timebomb. Armstrong is also a songwriter for other artists. Armstrong won a Grammy Award for his work with Jimmy Cliff and Pink, and has also worked with Joe Walsh and the Interrupters.

At the age of five, Armstrong met Matt Freeman while playing Little League Baseball. They grew up a few blocks apart in Albany, California, where Armstrong lived with his mother, father, and older brother, Jeff. Freeman and Armstrong formed bands many years later based on their shared love of bands such as the Clash and the Ramones. They both went to Albany High School.

Armstrong's relationship with Bikini Kill drummer Tobi Vail inspired the Rancid song "Olympia, WA" from ...And Out Come the Wolves.

He was married to musician Brody Dalle from 1997 to 2003. They met in 1995, when Dalle was 16 and Armstrong was 30, after Rancid and Dalle's band Sourpuss both played Summersault Festival in Australia. In 1997, when Dalle was 18, she moved to Los Angeles to live with Armstrong, and she formed the band the Distillers. The couple separated in 2003, after Armstrong saw a picture of Dalle kissing Queens of the Stone Age frontman Josh Homme in an issue of Rolling Stone magazine; she and Homme would later marry. Homme claimed he received death threats from Armstrong's fans. Dalle claimed Armstrong was very controlling of her and it took her three years to leave him. Some of Rancid's songs on 1998's Life Won't Wait ("Who Would've Thought", "Corazón de Oro") detail Armstrong and Dalle's relationship, and songs on 2003's Indestructible ("Fall Back Down", "Ghost Band", "Tropical London") deal with Armstrong's feelings about his divorce.

Armstrong's cousin, Scott, was the guitarist for Canadian punk band Desperate Minds, but they did not know each other until they were introduced at a show in Chicago in 1988 by John Jughead of Screeching Weasel