The FDJ rider beat Bryan Coquard (Europcar) and team-mate Nacer Bouhanni at the end of the 199km race with Benoit Drujon (BigMat-Auber93) making up an all-French top four.
Flamboyant Slovakian Peter Sagan (Canondale) was the favourite to win but the youngster abandoned the race.
The four escapees of Rudy Kowalski (Roubaix-Lille Métropole), Vegard Laengen (Bretagne-Séché Environnement), Flavien Dassonville (BigMat-Auber 93) and Sven Jodts (Colba-Superano Ham) dominated the first half of the French one-day race but the peloton reeled them in with 11km to go.
Cyril Lemoine (Sojasun) and Yannick Martinez (La Pomme Marseille) attacked to open up a five-second advantage with 7km of the race left, but were swept up by the chasing peloton and in the hectic bunch sprint Demare won out.
Seven of the last nine winners of this race have been French, with the now-retired Jimmy Casper claiming it five times.