Mignolet, 22, rose to prominence when helping his hometown club to a fourth-placed finish in last season's Jupiler League and was also chased by Dutch giants PSV Eindhoven.
But he opted for a move to Wearside where he will challenge Scotland number one Craig Gordon for a place in the starting XI.
The 6ft 4in stopper becomes manager Steve Bruce's second summer signing following the capture of midfielder Cristian Riveros, who is currently on international duty with Paraguay at the World Cup.
Bruce told the club's official website: "Simon is an excellent young goalkeeper and he has a lot more to offer. I'm sure he will push our current goalkeepers all the way next season.
"He is a player that we have watched on a number of occasions, as have a lot of clubs in Europe over the last couple of seasons.
"We're delighted to bring him to Sunderland. He'll come back ready for the start of pre-season training and will be looking to push for a place in the team."
Mignolet has signed a five-year contract and admits he was sold on the transfer after seeing the club's facilities.
"I am very proud to be here," he told SAFC TV.
"It is a great club with great supporters and a great stadium. I have seen it all and it was clear to me that I wanted to come here.
"I had a chat with the manager and the goalkeeping coach, and they told me what I can achieve here.
"The stadium, the facilities - everything about the club is massive. When you're a young kid like me it's impressive and I'm very happy to be here."
On the competition he will face to earn a starting berth at Sunderland, he added: "Competition is normal when you come to the Premier League. It is something you have to have in professional football.
"Of course I have to compete with other players and by doing that I will only get better.
"I am aware I'm only 22 and have a lot to learn. I think I can learn and progress in the time I am here."