10:17AM BST 01 Jul 2011
Sunderland confirmed last week that the 26-year-old Sweden international would be playing his football on Wearside next season, but his move would not officially go through until today following the expiry of his contract at St Andrew's.
Larsson joins Ahmed Elmohamady, Connor Wickham and Ji -Dong-won at the Stadium of Light with manager Steve Bruce having made significant early progress in his rebuilding plan.
Bruce signed the former Arsenal trainee for Birmingham for £1 million in Jan 2007 after a successful loan spell, and he went on to make 170 appearances.
However, he did not agree a new contract with last season's Carling Cup winners and, with a series of suitors on his trail, Bruce, who tried to sign him in January, has managed to get his man this time around.
The manager said: "Seb was a fantastic young player for me at Birmingham and he has matured into a very good midfielder.