Marc Demeyer (19 April 1950 – 20 January 1982) was a professional road racing cyclist from Avelgem, Belgium. He died of a heart attack at the age of 31.
Demeyer turned professional in 1972 for the Flandria team managed by Briek Schotte. He signed the contract while resting it on a car beside the start of Dwars door België, which he then won. Shortly afterwards he won the Grand Prix d'Isbergues.
Demeyer was one of the so-called "Three Musketeers", riding with and for Freddy Maertens and Michel Pollentier He led out sprints for Maertens in particular but could win them for himself, including stages of the Tour de France. He rode the Tour six times, finishing 72nd in 1973, 41st in 1974, 42nd in 1975, 56th in 1976, 49th in 1978 and 57th in 1979.
He won the intermediate sprints competition, known then as Points Chaud ('hot spot sprints') in the Tours of 1973 and 1975. He won two stages: the 19th in 1978 from Lausanne to Belfort, and the 14th in 1979 from Belfort to Evian-les-Bains. In 1974 he won Paris–Brussels and two years later Paris–Roubaix.