The French forward, 23, was involved in the accident in Pordic, Brittany at around 3pm GMT.
After beginning his career with Le Havre in Ligue 1, Julan made the move to Guingamp in January 2018.
He made 11 senior appearances for the side, with a loan spell at Valenciennes last season.
A Guingamp club 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.”
While Ligue 1’s official account also paid tribute: “We would like to offer our sincerest condolences to the family and friends of Nathael Julan and Guingamp.”
Guingamp, who are currently on Ligue 2’s mid-season break, have cancelled their friendly against Concarneau on Saturday.
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