Niagara Regional Police said the torso was spotted floating in the river on Wednesday afternoon.
Const. Derek Watson told CBC News in a telephone interview that "citizens saw what they believed to be a torso floating in the lower Niagara River between the Maid of the Mist and the Rainbow Bridge."
When pulled from the water, the torso was missing its arms, legs and head, Watson said.
Preliminary results from a post-mortem examination have determined that the victim is "a middle-aged Caucasian female," police reported in a news release on Thursday.
The news release said the victim had a pierced navel and "at least one caesarean section and a tubal ligation procedure."
Police expect to release further details about the victim on Friday, after an autopsy has been concluded.
Watson said police will be looking to the public to help identify the victim.
Identifying the victim is key," he said. "Over the next few days, as we're able to determine more things, we'll present it to the public to hopefully assist in indentifying her."
Watson said police are aware of missing people in Niagara Region, though he said there is no link to any cases "at this time."
Not related to Liu homicide
Police said this homicide is not related to the investigation into the death of Guang Hua Liu, the Toronto woman whose body parts turned up in two cities earlier this month.
Liu’s former boyfriend was recently arrested and charged with second-degree murder.
Another case involving a dismembered body also made headlines in Canada this year, when parts of a victim’s body turned up in Ottawa, Vancouver and Montreal.
Police would soon identify the victim as Jun Lin, an international student who had been studying at Montreal’s Concordia University.
His alleged killer, Luka Rocco Magnotta, was arrested in a Berlin café and subsequently extradited to Canada to face charges of first-degree murder, indignity to a human body and other criminal counts.