The 28-year-old was walking in Caledonian Road, near Alexander Street, in Wishaw, North Lanarkshire, when she was seriously sexually assaulted at around 12.30am on Sunday.
The suspect then left in the direction of Caledonian Road.
He is described as aged between 20 and 30, around 5ft 5in or 5ft 6in, of medium build with dark brown hair and dark clothing.
Officers from the National Rape Task-force are appealing for information and are particularly keen to trace two men who spoke to the victim outside Coopers Bar in Main Street.
Detective Inspector Helen Pasquale said: "I would like to appeal directly to the local community of Wishaw for information, and I would urge anyone who was in the vicinity of Caledonian Road and Alexander Street at the time of the incident to come forward.
"I would like to take this opportunity to reassure the local community that serious crimes of this type are rare, additional uniformed officers will be on patrol and I would encourage the public to contact the police if they have any concerns."
Meanwhile, 14 people appeared in court yesterday in connec-tion with a brawl, which broke out after a teenager's funeral on Monday night in Wishaw's Gateside Road.
Around 20 people were arrested, aged between 12 and 45, as reported in later editions of yesterday's Evening Times.
The disturbance took place after the funeral of 16-year-old Dylan Paton.