The attacking midfielder, who has put pen to paper on a five-year contract at St Mary’s, joins from Ligue 1 outfit Lille.
Morocco international Boufal is regarded as one of the most highly-rated youngsters in France.
The 22-year-old becomes the club’s most expensive player, eclipsing the £15m fee they paid for Dani Osvaldo.
He said: “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.
“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.”