The 31-year-old, who made 361 appearances for the Red Devils, was part of five title-winning sides at Old Trafford, and twice won the Champions League, FA Cup and League Cup.
Steve Bruce is set to make the most of his connections with his former club, with John O’Shea reportedly ready to follow Brown and a deal for midfielder Darron Gibson also rumoured to be on the cards.
Central defender Brown found his progress in at Old Trafford frequently interrupted by injuries, and fell behind the likes of Rio Ferdinand, Nemanja Vidic, Jonny Evans and Chris Smalling in the pecking order.
He retired from international duty after England’s 2010 friendly against Hungary having won 23 caps for his country.
Brown becomes Bruce's seventh signing of the summer as he cashes in on the recent sales of Darren Bent and Jordan Henderson.
The Sunderland boss told the club's official website: "We are delighted to welcome a player of Wes' ability, experience and character to the club.
"He has been a fantastic professional for Manchester United through the years and not only will he strengthen our defensive options, he will also bring with him a winning mentality and level of maturity which can only be positive for the players around him."
Sir Alex Ferguson has revealed it was a difficult decision to let Brown move to Wearside but believes it was the right time for him to leave Old Trafford.
He told Manchester United's official website: "Wes has been a great asset to the club during the most successful period in its history.
"He was a product of the youth system and marked himself out from an early age as one of his generation’s most natural defenders.
"In his time with us, he certainly became a favourite with the fans for his no-nonsense approach to the game and his infectious personality.
"It’s always the hardest of decisions to let loyal servants go. But life moves on and, unquestionably, Wes will get the chance to make a real contribution to Steve Bruce’s Sunderland and I’m delighted to be passing him into such good hands.
"Everyone connected with United wishes him a long and happy stay at Sunderland."