The club announced the transfer on its website, accompanied by some encouraging quotes by manager Brendan Rodgers:
I am delighted that we have been able to sign one of the top goalkeepers in the Barclays Premier League. Simon will join a club that will give him an opportunity to demonstrate and improve his huge talent.
Mignolet has spent the last three seasons with Sunderland while becoming one of the top young keepers in the world. While his club struggled and barely avoided relegation, things would have almost certainly been worse without the talented player defending the net.
In addition, the 25-year-old is a key part of the Belgium national team that is expected to make a deep run in next year's World Cup.
How will Liverpool finish in the EPL next season?
Liverpool hope that he will solidify a defense that tied for fifth in goals allowed this past league season. Considering the club already has an offense that finished fourth in goals scored, expectations are certain to be high for this historic organization.
Of course, the transfer brings a question about the future of Pepe Reina. The 30-year-old veteran has been with Liverpool since the 2005-06 season and has seen plenty of success over that time.
The addition of Mignolet has led many to speculate that Reina will be on his way out of Anfield. However, Rodgers has stated that he will not necessarily sell his current keeper, according to Pete O'Rourke of Sky Sports.
Graeme York of the Daily Mail also reported that Reina is happy where he is and is ready to compete for his starting spot.
While Mignolet was clearly brought in to play for the club, having extra talent at any position is never a bad thing.
Follow Rob Goldberg on Twitter for the latest updates in sports.