The 24-year-old worked under Sunderland head coach Gus Poyet at the Championship club and arrives on Wearside on a three-year contract.
Having started his career at Rochdale, Buckley made 68 appearances for the Lancashire club before making the step up to the Championship to join Watford in January 2010.
Buckley spent a season and a half at Vicarage Road, with Poyet then swooping to take him to Brighton in June 2011.
And Poyet has now signed Buckley, who scored 19 goals in 100 league appearances for the south-coast club, for a second time having also signed Liam Bridcutt from his former club in January.
Jack Rodwell, Costel Pantilimon, Jordi Gomez, Billy Jones, Patrick van Aanholt and Santiago Vergini have also arrived on Wearside ahead of the new season.
Poyet's men begin their Premier League campaign at West Brom on Saturday.
