The 22-year-old was attacked in Lochgilphead, Argyll, at about 12.45am on Sunday.
She was walking in a lane to the back of Manse Brae, near where it meets Thistle Way, when she was grabbed from behind.
Detective Inspector Kate Jamieson, of the National Rape Task Force, said: "The young woman is traumatised by what has happened and it is vital that we trace the person responsible. This type of incident is a rare occurrence in Lochgilphead and I want to reassure the local community that we are absolutely committed to identifying and arresting the man responsible for this attack.
"Significant resources are being used to support this investigation, and local people can be assured of an increased police presence in the area."
The rapist is white, between 5ft 8in and 5ft 9in and of medium build with dark brown hair. He was wearing a dark hooded top and blue jeans.