The Seahawks announced Monday that the charismatic coach has left Southern California after nine years to return to the NFL and to become the eighth coach in Seattle's 34-year history.
The Seahawks chief executive Tod Leiweke spent Sunday finalizing the contract for the 58-year-old Carroll.
Friday, the team fired coach Jim Mora following just one season. Seattle forced general manager and president Tim Ruskell to leave on Dec. 3. The moves left them rudderless less than four years after the Seahawks reached the Super Bowl.
"We are excited to add Pete as our coach. He brings a great passion for winning and a positive attitude that is contagious," said Seattle CEO Tod Leiweke in a prepared statement Monday.
The Associated Press contributed to this report.