The 25-year-old got into the taxi close to West Street in Sheffield city centre, South Yorkshire. She was taken to the Duke Street area of the city at 4.50am where she was attacked inside the car.
The victim is now being supported by specialist officers.
Temporary Detective Inspector Graham Stead, said: "We believe the woman got into a taxi from the city centre, close to West Street, and was taken to the Duke Street area, where the offence is said to have occurred in the car."
Now cops have launched an investigation into the horrific ordeal.
They are thoroughly examining CCTV footage across the city including the route they believe the attacker took.
He added: "An is now under way and we have a dedicated team of officers working on this.
"This is being treated as an absolute priority and we are already pursuing a number of strong lines of enquiry.
"A thorough review of CCTV footage of the area, including the route we believe the taxi took, is also now under way.
"The woman is being supported by specialist officers and I'd urge anyone who has any information about what happened to contact us.
"If you can help, or have any concerns, please call 101 quoting incident number 235 of 23 October 2016."
Information can also be given to Crimestoppers anonymously on 0800 555111.