Police were called to the scene just before 2:30 a.m.
They discovered the woman's body and evidence that she had died a violent death.
Montreal police spokesman Daniel Lacoursière said a man in his 40s was arrested in the apartment.
"He's going to be meeting with major crimes investigators later this morning, to determine what was his role in this seventh homicide," Lacoursière said.
Two children also in the apartment were found unharmed.