Le Havre defender Samba Diop has died at the age of 18.
The French second division club announced the news with "great sadness" on Saturday, without stating the cause of Diop's death.
Their Ligue 2 match with Reims - scheduled to take place later on Saturday - has been postponed.
The French football league has asked that all clubs in the country's top two divisions observe a minute's silence before matches this weekend.
A club statement said: "This is a horrible moment for all the players, staff members, volunteers, administrators and leaders."
Diop had played 21 times for the club's B team this season in the fourth tier of French football.
'The young hope of the club' - tributes
Yohann Thuram-Ulien, Le Havre goalkeeper: It is with great sadness that we learn of the death of Samba Diop, young hope of the club. He was generous and charming.
Alexandre Marfaing, Le Havre goalkeeper: Rest in peace my brother, a nice, good and respectful man. Now you're in heaven while yesterday you were by our side on the field... I'll miss you.
Benjamin Mendy, Manchester City and France defender: RIP Samba Diop.
AS Monaco: We offer our deepest condolences to the family, relatives and the club after the announcement of the death of Samba Diop.
French Football Federation: Our thoughts are with Samba Diop's loved ones and the whole club.