The woman, who is in her 20s, was attacked after two men offered her a lift in a car as she walked along High Street in the town.
She got into the vehicle in Holburn Avenue car park shortly before 02:00 BST on Saturday.
Police said the assault took place when the car drove towards Groomsport and stopped in a "dark area".
They have appealed to anyone with information about the attack to contact detectives.
The driver was described as thin, with a skinhead haircut and scruffy, ginger beard.
The second man was described as being of large build, approximately 5'4" tall, with a round face and brown, messy hair.
They were travelling in a silver car, possibly a Volkswagen Golf, in which the internal panel of the passengers' door was missing.
Police are investigating a serious sexual assault on a woman in Bangor, County Down.