The 25-year-old former Sunderland keeper completed his medical and signed on a long-term deal after making 90 appearances for the Black Cats since his debut in August 2010.
Mignolet, who has 13 Belgium caps, drew plenty of plaudits in England last season and will add extra competition for a starting berth alongside established goalkeepers Pepe Reina and Australian Brad Jones.
Manager Brendan Rodgers told the official website that his latest signing would be given opportunities to perform.
"I am delighted that we have been able to sign one of the top goalkeepers in the Barclays Premier League," said Rodgers.
"Simon will join a club that will give him an opportunity to demonstrate and improve his huge talent."
Mignolet said he was proud to become the first Belgian to be signed by Liverpool.
"I'm very confident in myself and I can't wait to get the first clean sheet under my belt," he said
Liverpool's latest addition is the fourth transfer to the English club in the close season. Striker Iago Aspas joined from Celta Vigo on Sunday, while Spanish Under-21 player Luis Alberto and former Manchester City defender Kolo Toure have also signed.
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