"I am very pleased that I can be at Anfield and a part of Liverpool for a long future," the 27-year-old, who arrived in 2013, told the club website (www.liverpoolfc.com) after signing the deal on Monday.
No details were given but the BBC and Sky Sports said it was a five-year deal.
Mignolet has missed just three Premier League games since 2013, making 122 appearances since he signed from rivals Sunderland.
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However, manager Juergen Klopp has had to defend him this season after occasionally scrappy performances by the Belgian, including last week's 3-3 draw with Arsenal.
"I want a keeper who helps, a keeper who comes out and tries everything," Klopp said last month of a player who also kept more Premier League clean sheets than any other goalkeeper in 2015.
Mignolet said his new contract came with added responsibility: "I'm getting older as well and coming into the years where people would say those are the years a keeper is at his best," he said.
(Reporting by Alan Baldwin, editing by Pritha Sarkar)
