Hamilton police say the victim in the city's 11th homicide was the suspect's 20-year-old girlfriend.
Police say Amy Hager, from Brampton, was living with the 39-year-old Hamilton man in a third floor unit at 907 King Street near Sherman Avenue.
It's unclear how she died, but her boyfriend is facing a second-degree murder charge.
Hager's family has been notified and police say they are "asking for privacy at this time to deal with the devastating news."
Officers say a 911 call was placed around 9:10 p.m. on Monday about the man needing medical attention in the area of King Street East and Sherman Avenue.
When paramedics and police arrived and found him, one of the officers recognized him as "someone he had dealt with previously," who lived in a nearby building.
Police started investigating how he became injured. They didn't comment on the nature of his injuries.
Det.-Sgt. Peter Thom with Hamilton Police Service told media that officers entered the man's apartment on the third floor at about 10 p.m.
"The way the male was found [outside] and the extent of his injuries, they felt there may be some other concern for other persons," he explained on Tuesday.
He said they found Hager suffering from signs of trauma. It's unclear how she was injured.
Paramedics couldn't save her life.
The major crime unit took over the investigation.
The suspect was in hospital on Monday and had a court appearance Tuesday.