The rule applies to anyone aged 12 years or older, "in public spaces, as well as on private premises accessible to the public."
The decision comes as cases in the region crossed the threshold of 50 daily cases per 100,000 people, the local government said.
Weekly infections in Belgium rose at the end of July and officials in countries across Europe are scrambling to prevent a second wave of the virus.
Belgium has 75,008 cases of coronavirus according to Johns Hopkins University.