Paramedics were called to an apartment on Sherbrooke Street East around 3:45 p.m.
They contacted Montreal police once they saw that the woman's injuries were suspicious.
The woman was rushed to hospital in critical condition and died later in the evening.
Officers with Montreal's major crimes unit arrested a 55-year-old man, who was in the apartment with the woman.
Police would not confirm how the man and woman knew one another.