The Black Cats have confirmed that the 32-year-old former Tottenham frontman has signed a three-and-a-half-year deal after completing a swap with United States international Altidore.
Defoe said: "I am delighted to be here and I'm looking forward to the challenge
It is a great club and I've always enjoyed coming up here and playing against Sunderland because of the stadium and the supporters
It is a great place to play and I'm really looking forward to it."
The move sees Defoe return to the Barclays Premier League after seeing out just 11 months of a four-year contract with MLS side Toronto
He could go straight into the squad for Saturday's trip to Spurs with head coach and former team-mate Gus Poyet is delighted to have secured the kind of firepower his side has been so sadly lacking to date this season.
Sunderland have scored just 18 goals in 21 league fixtures with Â£6.6million signing Altidore having contributed none of them, and Poyet will hope a man who managed 11 goals in 16 appearances for the Canadian side will help to address that failing.
He said: "I am thrilled that Jermain is a Sunderland player
I know all about his attributes and his qualities first-hand and his exceptional goal-scoring record speaks for itself
He has something different, something special.
"We know each other very well - he was very young when we played together and I was coming to the end of my career, so I know him from inside the pitch.
"When I returned to Spurs as a coach, it was a great experience for me to work with him, so I am really looking forward to having him as my player now and for him to be part of our team.
"I would also like to thank Jozy as his begins his career at his new club
It is a fresh start for him and we wish him well."
The Black Cats managed to beat off competition from a series of Barclays Premier League rivals, including Crystal Palace and QPR, to acquire Defoe's services, and sporting director Lee Congerton was thrilled to see his hard work finally come to fruition.
Congerton said: "Jermain is a player who has proven his quality in the Premier League
Naturally, with a player of his calibre, there was a lot of interest in him, not only in England but across Europe, so we are delighted that he has chosen to come to Sunderland."
Altidore, who joined the club from AZ Alkmaar during the summer of 2013, managed just three goals in 52 appearances in all competitions, and only one in 42 in the league.
Source : PA