Greece international Mavrias - known as 'Charis' - moves to the Stadium of Light on a three-year contract, becoming manager Paolo Di Canio's 11th signing of the transfer window.
Mavrias, 19, came through the academy at Panathinaikos, establishing himself as a regular with the Athens club after making his debut as a 16-year-old in 2010.
Capped twice by his country, Mavrias' progress through the ranks has been rapid, and he will now attempt to make an impression in the English game with a Sunderland side that finished a lowly 17th in the Premier League last term.
Di Canio has made wholesale changes in the close-season, but his team made a losing start to the new campaign, slipping to a 1-0 home defeat at the hands of Fulham on Saturday.
Sunderland travel to St Mary's Stadium at the weekend to face Southampton.
