The Brighton and Hove Albion winger, who worked under Gus Poyet with the Seagulls, has completed a move understood to be worth £2.5 million on a three-year contract
The 24-year-old worked under Black Cats boss Gus Poyet during the Uruguayan's spell at the Championship club and arrives on Wearside on a three-year contract. Goal understands that the deal is worth £2.5 million.
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 the Republic of Ireland-qualified star, who scored 19 goals in 100 league appearances for the south-coast club, for a second time, having also recruited 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.