The deal, which is understood to be worth around £11m, will officially go through when the window reopens on July 1.
Watson has agreed a four-year deal at the Amex Stadium, with an option for a further 12 months, but will remain available for selection at Sunderland for the remainder of the current campaign.
Brighton made attempts to sign Watson in each of the last two transfer windows, but Sunderland rebuffed the Premier League club’s approaches for their academy product.
However, with the 18-year-old set to enter the final year of his current contract this summer, [Black Cats](http://www.thenorthernecho.co.uk/sport/football/sunderland) chiefs have decided that this is the right time to accept Brighton’s latest offer.
Watson had shown no willingness to sign a new deal on Wearside, and the Sunderland hierarchy did not want to allow him to enter the final 12 months of his contract with his future uncertain.
They believe the £11m deal is a good one for a player who has only made seven senior starts in the whole of his career to date. The Black Cats are also understood to have secured a lucrative sell-on clause as part of the agreement with Brighton.
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Watson was a late substitute in last weekend’s 1-0 win over Millwall, and Sunderland officials are confident he will remain fully committed to the cause if selected by Regis Le Bris in the remainder of the season.
“Tommy’s a very talented young player, who’s been subject to multiple offers from Premier League Clubs over the past two windows, and it’s been well-documented that we’ve declined these approaches with the intent of extending his contract," said Sunderland sporting director Kristjaan Speakman.
"Unfortunately, this has not been possible, and we now believe this agreement represents the best long-term outcome for Sunderland AFC.
"Tommy first joined our club at age six and his rise to the first team demonstrates what is possible at the Academy of Light.
"We are proud of the role we have played in his development and although we will be disappointed to see him depart in June, we know he’s fully committed to helping the team throughout the final stages of the season.”