The 22-year-old netted 11 goals last season as Lille secured a fifth-place finish in Ligue 1, while Southampton’s outlay for the Moroccan surpasses the £15m they paid Roma for Dani Osvaldo in 2013.
“Sofiane is a player we have watched closely over the last few years and we’re thrilled to have him join the club,” said executive director of football Les Reed. “We’re delighted that Sofiane sees Southampton as the best place for him to continue his development.”
Surprise package Hull City, meanwhile, are also bidding to splash the cash and strengthen a squad which has won two of their opening three Premier League matches with only 13 fit senior players.
The Tigers are close to sealing a deal, which could be worth up to £5m, for Cardiff City goalkeeper David Marshall, while the club are in talks with Derby over a £10m move for midfielder Jeff Hendrick and Manchester United for striker Will Keane.