"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.
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)
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