"Fat Lip" is a song by Canadian rock band Sum 41. It is the fourth track on their debut album, All Killer No Filler (2001), and was released as the lead single in April 2001. It is the band's most successful single to date, topping the Billboard Modern Rock Tracks chart. It peaked at number 66 on the Billboard Hot 100 chart and at number eight on the UK Singles Chart.

Sum 41 is a Canadian rock band from Ajax, Ontario. The band was formed in 1996 and currently consists of Deryck Whibley, Dave Baksh, Jason McCaslin, Tom Thacker, and Frank Zummo.