The 33-year-old right-back was released from the final year of his contract with Lyon to join Lucien Favre’s men, who face Ajax in a Champions League qualifier next week.
Jallet, who has 14 France caps to his name and was a member of Didier Deschamps’ Euro 2016 squad, is understood to have penned a two-year deal.
The former Loreint player joined Lyon from Paris Saint-Germain in 2014 and made 92 appearances for the club. Over the course of his career, Jallet has played 308 times in Ligue 1.
Jallet is Nice’s fourth new recruit of the close season after Jean-Victor Makengo, Adrien Tameze and Pierre Lees-Melou, while striker Mario Balotelli was persuaded to sign a renewed contract after a successful 12-month stay for 2016-17.