The 59-year old was found half-submerged in the river Luye, close to the finish in the town of Gap. Guay was the uncle of one of the race competitors, Jérôme Pineau.
Tour de France organisers issued a statement on Tuesday saying that Guay was found dead next to his camping car close to the finish line in Gap." An investigation has been launched to confirm the cause of death," it said.
Guay was a cycling fan and a competitor himself in the 1970s. He had worked as a technician on the Tour de France for 27 years.
The Tour de France is in its final week and will end, as always, on the Champs-Elysées in Paris this Sunday.