The Seahawks issued a release saying Carroll, 58, agreed to a multi-year contract to be the eighth head coach in franchise history.
He replaces Jim Mora, who was fired Friday, apparently expressly to make room for Carroll to move in. Seattle was 5-11 last season -- Mora's only one as the Seahawks' coach -- after going 4-12 in 2008.
"We are excited to add Pete as our coach. He brings a great passion for winning and a positive attitude that is contagious," said Seahawks Chief Executive Officer Tod Leiweke. "We now turn our full attention to the hiring process for a general manager. Our intended structure is for Pete and the new (general manager) to work in a collaborative capacity on football matters."
It is Carroll's third stint as a head coach in the NFL. He coached the New York Jets in 1994, with the team going 6-10, and he led the New England Patriots to a total record of 27-21 in 1997-99.
USC was immensely successful under Carroll. The Trojans won seven Pac-10 Conference titles in his nine seasons and were a cumulative 97-19. USC was 13-0 in 2004 and won the Bowl Championship Series title.