New crash test standards are also being adopted that could both improve automotive safety and save car makers millions of dollars, Health Minister Leona Aglukkaq said Thursday as she took part in announcing the changes.
"Seat belts do save lives," Aglukkaq said. "Improving (seat belts) based on new tests and whatnot is important and we need to continue to do that."
On top of adding a mandatory requirement for lap and shoulder seat belts in the rear centre seat, new crash test standards are being adopted.
Car makers will have to use female and child-sized dummies in their crash tests, says Transport Canada.
Those tests must also be conducted at higher speeds for front-end crashes and improvements will be required in air bag deployment testing.
Canada has good auto safety regulations now, but more can always be done, Transport Minister Denis Lebel told the news conference at a department testing facility in Gatineau, Que.
"We'll continue to improve our safety (tests) day after day," Lebel said. "We have very strong regulations and we will continue to work on that."
The changes are being made to bring Canadian safety standards in line with those already adopted in the United States, under an agreement signed in February 2011 by Prime Minister Stephen Harper and U.S. President Barack Obama.
The move to align regulations will mean big savings for the automotive manufacturing sector, said Kash Ram, director general of Transport Canada's road safety and motor vehicle regulation division.
"Costs are expected to decrease because the manufacturers would not have to test to a unique Canadian requirement," Ram said. "Typically for any of these standards there are millions of dollars in expenses involved."
Older model cars will not have to be retrofitted with shoulder belts.
Note to readers: This is a corrected story. An earlier version said the change applies to new cars built in Canada.