"Chelsea Dagger" is a song by The Fratellis and their second single. It was released on 28 August 2006. It is taken from their debut album Costello Music, which was released in the UK on 11 September 2006. It is supposed to be named after both Jon Fratelli's wife Heather, a burlesque dancer whose stage name -- Chelsea -- he borrowed for the song, and also as a play on the name of pop singer Britney Spears.

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Characterised by its anthemic, scat chorus, it has been adopted on many football terraces as a crowd "favourite."

This song was played when Shunsuke Nakamura scored from a free-kick for Celtic Football Club in their 1-0 win over Manchester United on 21 November 2006 and was also played during Celtic's Scottish Premier League Celebrations after beating Kilmarnock 2-1. When Celtic score at home, the song's chorus is played. The Fratellis claimed on Soccer AM that this was a great honour and they hadn't known for a while, as they were too excited with the goal.

It was also played during Scotland's win over Georgia at Hampden Park in 2007 much to the delight of the Scottish fans.

It is played when Nottingham Forest, Ipswich Town, MK Dons, Rotherham United, Preston, Bristol City, Plymouth Argyle, Mansfield Town, Northampton Town, Coventry City, Brechin City and Middlesbrough score a goal (although many fans feel that the playing of this song and others detracts from the natural goal celebration). 

In ice hockey, it is played at the United Center for all Chicago Blackhawks goals and wins, and a version of the chorus is played on the organ during the listing of the game's "Three Stars". It is also played regularly at the Verizon Center for the Washington Capitals.