The 28-year-old was walking on 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/5ft 6in, of medium build with dark brown hair and dark clothing.
Officers from the National Rape Taskforce 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: "The victim is understandably obviously distressed and upset following her ordeal, and our specially trained officers are supporting her.
"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.
"It is important to stress that any information, however insignificant it may seem, may assist with our investigation.
"I'd like to take this opportunity to reassure the local community that serious crimes of this type are rare, additional officers will be on patrol and I would encourage the public to contact the police if they have concerns."