Barnsley have added to their attacking options with the signing of Mike Phenix from AFC Telford.
Former Barcelona goalkeeper Victor Valdes is training with Manchester United as he recovers from a serious knee injury.
A statement on the club's official website confirmed that Fernandez made the announcement to his players and the club's directors on Thursday.
Youth team coach Patrick Gabriel will take charge of the team on a caretaker basis.
Nancy prop up Ligue 1 with 11 points, eight adrift of safety, and only one win all season, which came on the opening day against Brest.
Fernandez's last match in charge brought them their first win in any competition since then, a 5-1 success against fifth-tier Dreux in the Coupe de France on Saturday.