The 28-year-old was attacked by a man as she cut through an underpass in Livingston, West Lothian between 3.20am and 3.45am on Saturday.
She fell to the ground and was subjected to a horrific sexual assault, police said.
It happened in an underpass off Grange Road at the east side of Perth Street, in the Craigshill area of the town, Lothian and Borders Police said.
When the attacker made off, the woman made her way home and then went to St John's Hospital where police were contacted.
The man is white, 25-40 years old and has a Scottish accent, with collar-length black or brown hair and is of a small build.
He smelled strongly of body odour and alcohol and was wearing a dark-coloured fleece, a dark-coloured beanie hat with a silver line on it and white canvas trainers.
Police are appealing for a potential witness with two black Labrador-type dogs, who is thought to have been in the area at the time, to come forward.
Detective Inspector Matt Paden said: "This has been a horrific attack for the victim to endure and she is now being offered all the support and assistance she requires from Lothian and Borders Police and our partner agencies.
"We are keen to speak with anyone who was in or around the underpass in the Perth Street area in the early hours of Saturday morning and remembers seeing anything suspicious.
"We are particularly keen to trace a witness who may have been in the area with two dark Labrador-type dogs and would ask this individual to contact police immediately.
"Similarly, anyone with any other information that can assist with our enquiries is also asked to get in touch."
Anyone with any information is asked to call Lothian and Borders Police on 101 or Crimestoppers anonymously on 0800 555 111.