Sporting Lisbon have confirmed the signing of Sebastian Coates on a five-year-deal.
The Uruguayan has impressed since moving to the Portugese capital in the summer, Jorge Jesus' side currently sitting third in the table behind league giants Porto and Benfica.
There had been much speculation that Sporting were preparing to activate a clause in his loan deal throughout January, and it is thought that Sunderland will land something in the region of £4.2 million.
That is over double what they are believed to have paid to sign him in 2014/15 season following a successful loan spell the previous season.
Fierce rivals Benfica had been linked with a shock move but Sporting have acted quickly to snuff out any interest. It further eases Sunderland's financial burden in terms of wages and transfer fees, after the sale of Patrick van Aanholt to Crystal Palace for £14 million.
Coates was a key part of the side that pulled away from trouble under Dick Advocaat, but fell out favour as Sam Allardyce preferred Younes Kaboul and Lamine Kone at the heart of defence.
He made 32 appearances in all competitions on Wearside.