Paddy McNair has completed his move to Middlesbrough.
The 23-year-old has signed a four-year contract with Championship side Boro, the fee is undisclosed but it is believed to be around £5million.
He becomes Boro's first signing of the summer and the first big-name player to leave the Stadium of Light, with the Black Cats squad undergoing a major revamp ahead of the League One campaign.
The box-to-box midfielder, a full Northern Ireland international with 20 caps to his name, scored four goals in the final five games of last season but his efforts were not enough to prevent Sunderland suffering successive relegations.
A Manchester United youth product, he was signed for Sunderland by David Moyes along with defender Donald Love for a combined £5.5million two years ago.
He made a total of 28 appearances in two years with Sunderland having suffered a cruciate knee injury in November 2016.
A Boro statement read: "The Ballyclare-born man returned from injury to end last season with a string of impressive performances, and four goals in his last five games further demonstrated his eye for goal.
"Due to the terms of the deal, the fee will remain undisclosed."
Brighton & Hove Albion had two bids rejected for McNair earlier this summer, the Seagulls unwilling to go above their valuation of £3.5million. There was also interest from Stoke City.
Tony Pulis was keen to land McNair to bolster his midfield options and they matched Sunderland's asking price, believed to be around the £5million mark.
A Sunderland statement added: "Sunderland AFC can confirm Paddy McNair has joined Middlesbrough for an undisclosed fee.
"The 23-year-old arrived from Manchester United in 2016 and went on to make 28 appearances for the Lads during a two-year spell.
"Everyone at the club would like to wish Paddy the best of luck for the future."
Sunderland owner Stewart Donald spoke about McNair's future on the Roker Report podcast last Thursday. Donald said: "Has Paddy turned in a transfer request? No.
"Has his agent told us that Paddy McNair will not play for the club again? Yes.
"Not one Premier League club has reached the valuation. Middlesbrough and Stoke are close.
"Brighton don’t see him past £3million. If we say it was £5million or whatever, the market now knows we have accepted an offer for £5m."
McNair's move to Middlesbrough looked to have collapsed last week when the Teessiders withdrew their offer over suggestions they were unhappy the midfielder was still being touted to other clubs.
Donald flatly refuted any claims Sunderland have been touting McNair around, adding he believed it was McNair's representatives. The deal was resurrected over the weekend and McNair completed his medical on Monday ahead of tonight's official announcement.