The American striker will complete his move following a physical examination on Monday, according to the player's personal website in a post that was later removed. Goal first reported that Sunderland made a bid on Wednesday.
Altidore, 23, netted 51 goals in 93 games for AZ - including 31 in 40 games during the second year of his two-year stint in the Netherlands. He had the winning goal as AZ lifted the KNVB Cup and also set a new goal-scoring record for an American playing in a European first division.
The U.S. international has also been in hot form for his national team, breaking a scoring drought with four goals in the month of June.
His new club, Sunderland, struggled last season under head coach Martin O'Neill, leading to the Northern Irishman's dismissal and the appointment of the controversial Paolo Di Canio. The Italian led the club to a 17th-placed finish, one above the relegation zone.
Altidore's only previous experience with English soccer came during a season at Hull City, on loan from Villarreal, where he scored just one league goal in 28 appearances.
He is the second promising youngster to exit AZ in recent days. Playmaker Adam Maher joined PSV on July 1.
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