The 22-year-old has signed a five-year deal with the Saints after an impressive season with Lille last year.
The Morocco international scored 11 goals last season as Lille qualified for Europe.
Southampton Executive Director of Football Les Reed said on the club's official website: "I’m absolutely delighted to welcome Sofiane to Southampton Football Club.
"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.
"Sofiane is a player we have watched develop closely over the last few years, and we’re thrilled to have him join the club ahead of what promises to be a very exciting season, with Europa League football on the horizon".
Boufal also said that he was "very, very happy" to have joined the Premier League club.
"I’m very, very happy to sign for Southampton and I am really excited to play in St Mary’s Stadium in front of the fans," he said
"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".