The Saints agreed a fee last week for the Lille forward, which it is thought to have topped the £15m they paid for Dani Osvaldo in October 2013. Boufal passed a medical with the club on Monday.
After signing a five-year contract, the 22-year-old Moroccan international said: “I’m very happy and really excited to play in St Mary’s Stadium in front of the fans.
“Southampton showed big interest in signing me and I can see that this club is the best place for me to continue my progression as a footballer.I hope I can achieve many great things with Southampton. It is a very good club, with excellent facilities, and I feel it is the perfect environment to continue my development.”
Southampton’s executive director of football Les Reed said: “I’m absolutely delighted. He has shown his undoubted qualities in France over the past few seasons, and we are extremely confident he will successfully adapt to life in the Premier League.
“At just 22 years of age, he will undoubtedly continue to improve as a player, and we’re very pleased that he will be able to make that progression with us here at Southampton.”
Boufal impressed for Lille last season by scoring 11 times and adding five assists, but he picked up a knee injury in May and is not expected to be able to return to action until next month.
Southampton have lost forwards Sadio Mane and Graziano Pelle over the summer, although they managed to bring in Nathan Redmond from Norwich.