The incident happened around 3am to 3.30am yesterday, when a 24-year-old woman was approached by an unknown man and sexually assaulted in an alleyway near Grand Central Nightclub in Livingston.
The suspect is described as black, 5ft 6inches - 6ft tall, slim build, with dark dreadlocks. He was wearing a blue top or jacket.
Officers are now urging anyone who can assist with their enquiries to come forward.
Inspector Samantha Ainslie said: “We treat all reports of sexual assault very seriously and are currently pursuing various lines of enquiry to find the perpetrator.
”We’re appealing for anyone who witnessed the incident, who saw the suspect around the time of the incident, or who knows who he is, to come forward.”
Anyone with information is asked to contact Police Scotland on 101 or contact Crimestoppers anonymously on 0800 555 111.