The shooting happened at about 9:30 p.m. near the intersection of Summit Avenue and Sixth Street.
The woman was in critical condition when she was taken to Allegheny General Hospital in the North Side. Her condition has been upgraded stable, officials said Wednesday morning.
The front yard of a home in the 600 block of Summit was cordoned off with police tape and a black SUV was towed from the scene, police said.
The woman was not identified and no information was available about what may have led to the shooting.
The incident is being investigated by Monessen police and Westmoreland County detectives.