Officers said they received a 911 call shortly after 3:45 a.m. about a woman who was injured inside her home.
Police said the victim, a woman in her 30s, was rushed to hospital, where she was declared dead.
Two children were found inside the home — a 12-year-old girl and a 2-year-old boy — but were not injured.
Longueuil police Const. Mark David said it was the young girl who phoned 911.
Officers have set up a perimeter outside the house near the intersection of Dandurand and Drapeau streets, where they are investigating.
There are no suspects so far.