It happened at a house on Newry’s Hollyridge Lane, between the Fiveways roundabout and Carnbane industrial estate.
Police said it is not yet clear how he she sustained her injuries.
Officers found her as they followed up a report about an intoxicated man at the roundabout.
The woman was initially taken to Daisy Hill Hospital in the city but was later transferred to Belfast’s Royal Victoria Hospital.
Det Insp Colin Patterson said her condition “has been described as life threatening”.
“We are investigating the cause of her injuries and the circumstances surrounding this incident,” he said.
Newry and Armagh MLA Danny Kennedy said it was “deeply disturbing news”.
“This incident will shock the community and I would appeal to anyone with any information to contact the PSNI immediately.”
Sinn Fein Newry MP Mickey Brad said local people were “shocked” at the news.
Anyone with information is asked to contact detectives at Ardmore PSNI station in Newry on the 101 number.