The 23-year-old was involved in a car accident on the road to Pordic, near the town of Saint-Brieuc.
A short statement read: “The club had the immense pain of learning this afternoon of the accidental death of their player Nathael Julan.
“On this tragic day all the members of the club join together to send their condolences to Nathael’s family.”
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Guingamp have since cancelled a scheduled friendly with Concarneau on Saturday.
Julan began his career with Le Havre before joining Guingamp in 2018.