It happened at the Carr Road, Lisburn area in the early hours of Tuesday 16th September.
Detective Sergeant Gary Chambers said: “At around 1:30am, it was reported that a 19 year old female got into a taxi outside licenced premises at the Lower Crescent, South Belfast area.
“The female is described as being 5″6, long dark hair and was wearing a pink skirt and grey vest top.
“It is believed the driver of the vehicle took the female to the Carr Road, Lisburn area. At around 3:40am, it was reported to police that the female passenger was assaulted.
“Enquiries are continuing and police would like to speak with a male passenger who was in the taxi but was dropped off to his destination prior to the incident, who might be able to assist with their enquiries.
“Anyone who saw any suspicious activity in the Carr Road area prior to 3:40am or the passenger of the vehicle is asked to contact detectives on the non emergency number 101 quoting reference 99160914.
“Or if someone would prefer to provide information without giving their details they can contact the independent charity Crimestoppers and speak to them anonymously on 0800 555 111.”