She was taken to hospital in critical condition and later died.
Neighbours have the woman was stabbed, but police have not confirmed that.
At least three people were taken into custody but no charges have been laid yet, police said.
Jason Burton, who lives in the unit next door, says he was out having a cigarette on his balcony at about 10:30 p.m. when he heard a commotion.
“I heard screaming next door, but I didn’t think too much of it. I figured it was just a couple having an argument,” he said. “I went back in and next thing I know there’s cops showing up everywhere.”
“I saw this gentleman, he was crying, the cops were parked just up the road there, and they brought him out in handcuffs,” Burton said.
“And there was another young kid … it looked like he had his arm sliced open. There was an ambulance here, and they were attending to him.”
Police said it’s not believed the incident was random. An autopsy is set for later on Wednesday.
Anyone with information is asked to call police or contact Crime Stoppers. Homicide detectives are investigating the death of a woman after an assault in southeast Calgary. Police were called to the four-unit apartment building in the 1800 block of 42nd Street