Middlesbrough boss Gareth Southgate has expressed his delight at capturing Julio Arca.
The Argentine, who can play in midfield or at full back, has penned a five-year deal after Boro agreed a £1.75 million fee with The Black Cats.
Southgate admits he was quick to make his move for the 25-year-old.
"As soon as I discovered that Julio was available it didn't take us long to make a decision," said Southgate.
"In the games against Boro last year, Julio showed us exactly what he can do so I am delighted to be able to bring such a top quality player to the club.
"The thing about Julio is that he symbolises the way I want to play. I want a team that thinks and that attacks, and Julio has shown me on the occasions that I have played against him that he is an attacking player.
"He can play in midfield or he can play at left back, and I want my full backs to push forward this season. Julio can do that.
"Julio has been very loyal to Sunderland for the last five years and I know that he has the right kind of character that I want to bring into the dressing room.
"He is also a player who excites the fans and can make us a better club."
Arca was pleased to make the switch from Wearside to Teesside.
"Last season was hard for everyone and it was very disappointing to go out of The Premiership," Arca said.
"When Middlesbrough came in, obviously I wanted to listen. This is a big club and these opportunities don't come along every day.
"I spent several years at Sunderland and I was very happy there, but you have to do what is right for your career."