The Liberal government's Bill 62 on religious neutrality was passed Wednesday in Quebec's National Assembly.
"We are just saying that for reasons linked to communication, identification and safety, public services should be given and received with an open face," Couillard told reporters.
"We are in a free and democratic society. You speak to me, I should see your face, and you should see mine. It's as simple as that."
The law prohibits public workers — including doctors, teachers and daycare employees — as well as those receiving a service from the government from covering their faces.
It was extended to municipal services, including public transit, in an amendment made in August.
The Liberals, who hold a majority in provincial parliament, voted in favour of the bill, while all the other parties voted against.
The two main opposition parties, the Parti Québécois and Coalition Avenir Québec, have argued the legislation doesn't go far enough, while civil rights advocates and Muslim groups argue it discriminates against religious minorities.
While the niqab and burka are not mentioned in the legislation, Quebec Justice Minister Stéphanie Vallée said earlier this week the bill aims to clearly establish the "neutrality of the state."
The bill provides for the possibility of religious accommodation in certain cases, leading critics to question how far reaching it will actually be.
It remains unclear how religious accommodation requests will be evaluated.
The province will work with public services, including municipalities, schools and public daycares, to establish guidelines for how it will be enforced, Vallée said. Those may not be ready until next summer.
Montreal Mayor Denis Coderre, who is in the middle of a municipal election campaign, has been an outspoken critic of the bill, accusing the provincial government of overstepping its jurisdiction and ignoring his city's multicultural character.
On Tuesday, he said there are "serious problems" with how it would be applied.
Vallée tabled the legislation in 2015, a year after the Liberals took power.
Religion and identity became a key issue in the last election after the Parti Québécois put forward a contentious proposal for a charter of values, which would have banned public servants from wearing obvious religious symbols.
The National Council of Canadian Muslims issued a statement saying it was "deeply concerned" by the new law and that it "boils down to ugly identity politics" ahead of the 2018 provincial election.