The 28-year-old was attacked as she cut through an underpass in the Craigshill area of Livingston, West Lothian, between 3.20am and 3.45am.
She fell to the ground and was subjected to a horrific sexual assault, 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 and dark-coloured beanie hat.
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.