The 21-year-old only joined the Latics from Middlesbrough last summer for £3.5 million.
And although the England U21 international was linked with moves to Tottenham Hotspur, Arsenal and Liverpool, he has now chosen to follow former Wigan boss Steve Bruce to the Stadium of Light.
"The fee is a good one, it is far more than the club paid for him last year and the third highest in Wigan Athletic history," new manager Roberto Martinez told the club's official website.
"The initial offer from Sunderland was unacceptable as we said at the time, but they have now matched our valuation.
"The speculation about Lee has refused to go away over the last few weeks and this has been very unsettling for him and for our players.
"It became clear over the past days having spoken to Lee that he wanted to leave and it is very disruptive to the group when a player is clearly unhappy.
"My number one priority has to be the good of the club and it left me with a decision to make, I therefore agreed to the move for the good of the players we have at the club and who want to remain with us and help us progress to the next level."
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