The Spaniard sealed a move to the Stadium of Light as his contract with Wigan Athletic expired at the end of the season following five years with the Latics.
"I'm really happy to be here and join such a big club like Sunderland, and to have this new challenge in my career," he told the club's official website.
"I'm looking forward to playing here. I've heard good things about the fans and how they help the team with a big support.
"Last season they played a big part in the good results the team had. I've played at the Stadium of Light and always felt the support was brilliant."
Gomez will officially join Sunderland on July 1.