The new law carries a penalty on conviction of a $200 fine or as much as a week in jail, Deutsche Welle reported.
The imam of the main mosque in Brussels, Mustafa Kastit, told the German broadcaster the government had spent a lot of time and money on a minor issue.
"You can go walk in our Muslim neighborhoods here and hardly see any women wearing a niqab," he said. "So is it really worth legislating against them?"
Census figures show there are about 400,000 Muslims in Belgium. Muslim clerics say only 270 women wear the veils or full body covering.
The law passed with near-unanimity in parliament, with Green Party Eva Brems the lone dissenter.
"I am convinced it is a violation of human rights," she said. "I hope it will be challenged and annulled."
France became the first European country to ban face-covering in April.