For Belgium itself? No.

For Flanders there's Hendrik Conscience's The Lion of Flanders, which is about the Franco-Flemish war and specifically the Battle of the Golden Spurs.

There's also Van den vos Reynaerde (Of Reynard the fox) which is more of a Dutch language "national" epic.

I'm drawing a bit of a blank on Wallonian ones.

Wallonia and Flanders have The Four Sons of Aymon and the Horse Bayard, a chanson de geste (an epic poem) first attested in the 12th c., in French. The setting ranges from the Walloon cities of Dinant and Namur, the Ardennes, Gascony and finally Cologne. It recounts the persecution by Charlemagne of the eponymous Sons and their horse.

The poem cycle (there were spin-offs) is pretty international in its settings and its popularity. There are versions in Dutch, German, English and Italian, so many European countries could claim it. The origin stamps it as French but most French people would say that the Chanson de Roland is their pre-eminent founding epic. Germans probably would point to the Nibelunglied and the English would certainly claim La Morte d'Arthur as their founding epic. So Wallonia and subsequently Flanders seem to have chosen the Sons of Aymon as their traditional epic/mythical poem cycle.