The 21-year-old Frenchman previously played under Saints manager Claude Puel at the Ligue 1 club. "I am really happy," said Hassen. "It's a pleasure for me to come to England and I am really happy to get to know the Premier League." Southampton's executive director of football Les Reed added: "He has built an excellent reputation during his time with Nice."
Hassen, who has made 50 first-team appearances for Nice, has represented France at every youth level, from under-16s through to under-21s. The Saints are also close to completing a £14m deal for Napoli forward Manolo Gabbiadini.
