Saint John Police say it broke out at around 2:30 a.m. Sunday on Prince Edward Street. The building had a small business on the ground floor and apartments above it.
There are no reports of any injuries.
Saint John Police Sgt. David Cowan says firefighters are working to save two buildings next door.
He says smoke is blowing into a 10-storey building across the street and most of the people living inside it have left.
Cowan says they are mostly senior citizens. They're being sheltered in a nearby mall.
Joshua Kindred lives two blocks away.
“When I went up there, there were no flames. The buildings were burnt and gone and fell to the ground by the time I got there. They’re still dumping water on it right now and we have no power. All the power lines are down. So yeah, they’ve got city transit buses for people to sit in, but all our power is out and our house is starting to get cold already,” he said.
Several streets are closed as firefighters continue to deal with the fire, including Prince Edward Street, Union Street and Waterloo Street.