Officers responded to a report that a person was shot in the area of South Second and East San Salvador streets in downtown at about 1:20 a.m., Sgt. Jason Dwyer said.
The victim, believed to be a 19-year-old woman, was transported to Regional Medical Center where she was pronounced dead, Dwyer said.
An investigation revealed that the victim and a 21-year-old man were driving in the area of South Third and East San Salvador streets when the victim was shot by someone outside the car, the sergeant said.
The driver, who was not hit, stopped the car and called 911, Dwyer said.
Dwyer said that no suspects have yet been identified or arrested. It is unclear why the shooting took place and whether it was gang-related, he said.
No one else was injured in the shooting.
Police are continuing to investigate the incident.
The homicide marks the city's 30th of the year.