Currently, only lap belts are required in that position in Canada, but shoulder belts in the middle rear seat are already standard equipment in some vehicles.
"Making shoulder belts mandatory in the rear centre seat will reduce the number of deaths and serious injuries on our roads," Transport Minister Denis Lebel said in a statement.
Owners of older vehicles will not have to retrofit their cars to comply with the new rule.
Vehicle manufacturers are throwing their support behind the move.
"The revisions to this regulation will ensure not only enhanced safety protection for occupants in frontal crashes, it will also more closely align both Canadian and U.S. vehicle standards making advanced safety technologies more readily available and affordable," said Mark Nantais, president of the Canadian Vehicle Manufacturers' Association.
All vehicles in the U.S. have been required to have lap and shoulder seatbelts for the rear centre seat since the 2008 model year.
Also Thursday, Transport Canada announced tighter vehicle testing rules, including requirements that:
- Vehicle safety tests use female and child-sized dummies, in addition to the male test dummies that were previously required.
- Air bag deployment testing be improved.
- Vehicle test speeds be increased to better protect occupants involved in serious frontal crashes.