Firefighters were called to a home just east of Regina's downtown in the early-morning hours on Tuesday.
Deputy fire chief Gerard Kay said a passerby saw the flames and pulled the woman and child from the home.
"I know the neighbours are very saddened," Kay said. "She was very loving and really took care of this young boy."
Marilyn Tanner, 58, and her one-year-old grandson, Alonzo Tanner, were pronounced dead in hospital.
Kay said investigators were trying to determine if there was a smoke alarm in the house.
"Our bylaw states that if it's a rental property, it must have a hard-wired smoke alarm," he said. "We have not found one as of yet."
It's important for officials to check if the family was properly protected, Kay added.
"This fire grew to a very large proportion of the home, it appears, before these people got out.
"I arrived on scene and the whole entire structure was gone and I was maybe half an hour into that scene."
Police spokeswoman Elizabeth Popowich said the fire damaged a neighbouring house, but no other people were injured.
She said the fire was "well underway" when emergency responders arrived.