Sunderland signed defender Adam Matthews from Scottish Premiership champions Celtic for £2million.
The 23-year-old full-back has signed a four-year deal with the Premier League club, who announced the move on Friday.
Sunderland coach Dick Advocaat was pleased with the addition and said 12-time Wales international Matthews would add quality to his squad.
"We know that we need quality players to improve competition for places in the side, so Adam is a welcome addition to our squad and we are very pleased to welcome him to Sunderland," he said.
Matthews started his career at Cardiff City and made over 40 senior appearances for the club before moving to Celtic.
Sunderland sporting director Lee Congerton added: "Adam is a player we have been tracking for some time.
"Even though he is only 23 years old, he has a wealth of experience, and I am sure he will continue to improve under the leadership of Dick Advocaat."
