Sunderland have signed Greece international winger Charis Mavrias from Panathinaikos on a three-year-deal, the Premier League club said on Thursday.
The 19-year-old has been capped twice by his country, and in 2010 became the second-youngest player to appear in the Champions League when he was 16.
He made 70 appearances for Panathinaikos after coming through its youth academy, scoring six times.
No transfer fee was disclosed, although media reports put the figure at 2.5 million pounds ($3.92 million), and he becomes the 11th player signed by manager Paolo Di Canio during the transfer window.
Sunderland lost its first Premier League game of the season at home against Fulham and travels to Southampton for its next match on Saturday.