Buckley has signed a three-year contract. While Sunderland did not disclose the transfer fee, British media reports suggested that he had cost them in the region of �2.5 million ($4.2 million, 3.1 million euros).

The move reunites Buckley with Sunderland manager Gus Poyet, under whom he previously played at Brighton.

He is Sunderland's seventh signing of the close season, following the arrivals of Jack Rodwell, Costel Pantilimon, Jordi Gomez, Billy Jones, Patrick van Aanholt and Santiago Vergini.

Sunderland launch their Premier League campaign away to West Bromwich Albion on Saturday.