The 31-year-old has signed a four-year deal with the Black Cats, bringing an end to a long career at United.
After making his first-team debut in May 1998 at home to Leeds, Brown went on to make 362 appearances and score five goals for the Reds, despite some terrible luck at times with injuries. He leaves with five league championship medals plus two winners' medals apiece in the Champions League, FA Cup and League Cup.
Sir Alex Ferguson hailed Brown's contribution to the club, telling ManUtd.com: "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."