Emergency response officials told AFP that one person was transported to hospital, as local media posted images of several fire trucks, their long ladders extended and hoses strewn across the road at her downtown residence.
The prime minister's office did not immediately respond to requests for information.
But public broadcaster Radio-Canada said Margaret Trudeau, who is also the widow of late prime minister Pierre Trudeau, was treated in hospital for smoke inhalation and possibly burns.
She is expected to recover.
The blaze reportedly started on a fifth floor terrace, and took 70 firefighters to douse. Three families were evacuated.