The Belgium goalkeeper is Brendan Rodgers' fourth summer signing, as he aims to bring back to glory days at Anfield after years of disappointment and failures in the Premier League.
The 24-year-old has long been admired by Liverpool and they were strongly linked with him back in January, but no deal could be finalised.
Mignolet's future was a hot topic this summer, with both Liverpool and Arsenal strongly interested in the shot-stopper, but he has finally made the move from the Stadium of Light to Anfield.
The signing of Mignolet will raise further questions about the future of current number one Pepe Reina, who is strongly linked with a move to Spanish giants Barcelona.
However, the goalkeeper himself has revealed that he welcomes the signing of Mignolet.
"I said it many times I am really comfortable and very happy in Liverpool," Reina commented. "Every team needs competition. It is more than welcome, and if it is finally done I welcome him and anything he needs, I'm here."
Liverpool manager Brendan Rodgers have revealed that he is delighted to have signed the Belgium goalkeeper.
"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."
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