"L'En Avant Guingamp is in mourning," said the club. "The club had the immense pain to learn this afternoon of the accidental death of its player Nathael Julan.
"On this tragic day, all members of the club join together to express their sad condolences to Nathael's family."
It did not give any further details. French media, including the website of L'Equipe, said he died in a car crash after a training session.
Julan was raised at Le Havre where he made his professional debut in Ligue 2 in 2015.
He went on to make 42 appearances for Le Havre, scoring nine goals, before joining Guingamp in January 2018 when they were in Ligue 1.
He made 10 top flight appearances before being loaned to Valenciennes in January last year and returned to Guingamp in June. Guingamp were relegated last season.