The Belgium international has penned a "long-term" contract at Anfield after passing medical.
The 25-year-old has impressed in the North East since arriving from Sint-Truiden in 2010, establishing himself as one of the most promising keepers in Europe.
It's a big club and when you arrive it's a big thing," Mignolet told liverpoolfc.com.
"I'm very pleased to be here and I'm looking forward to getting started.
"The first training [session] will be very early and I'll be happy to be there.
"I think it will be very special the first moment I step onto the pitch."
Mignolet becomes Liverpool's fourth signing of the summer following Manchester City's Kolo Toure, Celta Vigo's Iago Aspas and Sevilla's Luis Alberto.
Manager Brendan Rodgers is keen to get his business done early and he is also tracking Sporting Lisbon defender Tiago Iloris and is interested in Shakhtar Donetsk midfielder Henrikh Mkhitaryan.
He is also interested in signing another centre-back to ensure he has enough cover following the retirement of Jamie Carragher.
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