The Saur-Sojasun rider claimed his fifth victory of the year as he beat Greg Van Avermaet (BMC) to the line.
Six riders broke clear on the climb to the citadel at Namur with Jan Bakelants, Björn Leukemans and Bert De Waele joining Belgian compatriot Van Avermaet, Simon and Italian Luca Paolini.
But it was the 26-year-old from Rennes who had the speed at the death as the course flattened out as they passed under the Flamme Rouge.
A 25-man breakaway formed on the 203km route, which featured six climbs, and they built up a two minute lead.
But they were caught in instalments with triple world champion Oscar Freire's bid ending towards the finish.
A counter attack by Team Europcar's Yukiya Arashiro was foiled with 5km remaining and a large group came to the foot of the final climb together.
Tom Boonen was looking for the victory in his final competitive outing ahead of the world championships road race in Valkenburg on September 23.
But he lost contact on the steep ramps inside the final 3km and it was left to Simon to claim victory following on from successes this year in the Tour du Finistère, Grand Prix de Plumelec-Morbihan and Volta a Catalunya.
- Greg Van Avermaet
- Bert De Waele
- Jan Bakelants