The 25-year-old shot-stopper made 101 apearances in three years for the Black Cats, but has swapped Wearside for Merseyside after penning a "long-term contract" following a successful medical at Melwood.
Mignolet was one of Sunderland's star performers in a largely unspectacular season for the club, and Reds boss Brendan Rodgers believes the club have acquired a "huge talent."
"I am delighted that we have been able to sign one of the top goalkeepers in the Barclays Premier League," Rodgers told the club's official website. "Simon will join a club that will give him an opportunity to demonstrate and improve his huge talent."
The signing of Mignolet will add further fuel to the speculation linking current Liverpool No. 1 Pepe Reina with a move away from Anfield, but Rodgers has previously insisted the Belgian has been bought to provide competition for the Spaniard.
Sunderland boss Paolo Di Canio was sad to loose the young goalkeeper, and he thanked Mignolet for his contribution to the Black Cats' survival bid last season.
"Simon is a fantastic footballer, a fantastic professional and a fantastic lad and I would like to personally thank him for his contribution to the team last season," Di Canio told Sunderland's official website. "He was crucial to what we achieved and he goes with the very best wishes of everyone at the club."