Dutch (often called Vlaams or Flemish in the Belgian context) is the official language of the Flemish Region (Flanders), which comprises five provinces in northern Belgium:
Brussels-Capital Region is officially bilingual (Dutch/French). While French is the majority lingua franca, a substantial Dutch-speaking minority resides there, and all public services are legally required to be available in both languages.
Dutch is not an official language in the Walloon Region (Wallonia), where French (and German in the East Cantons) prevails.