Bill 3 provides for a 50-50 split between municipalities and unionized workers on pension contributions and deficits.
Amendments sought by the Parti Quebecois and agreed to by the Liberals open the door to workers paying between 45 and 50 per cent, with municipalities paying between 50 and 55 per cent.
If the two sides can't reach an agreement, the 50-50 scenario will apply.
The government says the legislation was necessary to help municipalities grappling with pension deficits that, across the province, could total as much as $3.9 billion.
The Quebec Federation of Labour said it is considering a legal fight, arguing the law is unconstitutional.
The secretary-general of the labour group called the law's passing "a day of mourning" for the municipal world.
"Not only has the Liberal government of Philippe Couillard remained insensitive to the demands of workers and retirees in the municipal sector, but he also refuses to make public the status of pension deficits, as if he wanted to hide the truth from the people," Serge Cadieux said in a statement.
Cadieux said the actual municipal pension deficits are half of what they were in 2012.
Municipal Affairs Minister Pierre Moreau appeared undaunted by threats of court challenges and instead called on unions to put their energy into negotiating.
"A principle in our society is the presumption of the validity of a law," he said.
"We are comfortable with the idea that this bill meets all the requirements to successfully withstand any court challenges."
The government had hoped to pass the bill before Christmas and Moreau said he's happy it went through with only limited delays.
"I am very pleased that we have been able to adopt the bill," he said. "I think it reflects a broad consensus in society."
Bill 3 triggered widesread protests among municipal workers over several months and unionized workers are promising to continue their battle.
"We have a cause that's deeply just and we'll go on right to the end," said Marc Ranger, a spokesman for the main coalition fighting the legislation.
Members, he noted, are as outraged as ever and their movement remains strong.
Moreau said better results in the markets suggest the $3.9-billion deficit may now be closer to $3 billion. The union pegs the number at about $2.2 billion.
But even the union figures would suggest a "troubling" future for those pensions, Moreau added.