The Belgian, who has previously shown his speed by winning the Trofeo de Palma Mallorca and the Handzame Classic, sealed a fine victory in Ingooigem.
Argos-Shimano’s Luka Mezgec could not quite match Dehaes, with last year’s winner Nacer Bouhanni also outsprinted.
It was the third bouquet of flowers of the season for the 28-year-old from Brussels, in the 66th edition of the race.
The climax of the race followed a 12-rider breakaway after 10km as the group built up a 5:30 advantage, but the early movers could not sustain their advantage.
Omega Pharma-Quickstep’s Tom Boonen could not finish the race and the Belgian, wearing the champion’s colours for the last time, later revealed that he had not fully recovered from the Tour de Suisse.