The 19-year-old winger, who has been capped twice by Greece, has signed a three-year deal with Paolo Di Canio's side
The 19-year-old winger has signed a three-year contract with Paolo Di Canio's side.
Mavrias, who has been capped twice by Greece, scored six goals in 70 appearances for Panathinaikos.
"I know him, I know that he is a very good prospect, I know that he’s a very exciting player," Di Canio told reporters.
The midfielder is Sunderland's 10th signing this summer as Di Canio's men look to improve on last season's 17th-placed finish in the Premier League.
Mavrias has been given the number 35 shirt for the campaign.