The alleged victim was approached by a man at 9pm last night in the underpass beneath Wester Hailes Road in the west of Edinburgh.
He spoke to her then sexually assaulted her before she ran off towards Murrayburn, police have said.
The suspect is white, aged in his mid-20s to early-30s, has dark hair, is 5ft 8in, has an athletic build and his nose seemed to have been broken. He was wearing a white hooded top with an emblem on the left chest.
Detective Sergeant Keith Fairbairn said: "While the victim was not injured, she was left understandably distressed as a result of her ordeal, and we are appealing for any information that can help us trace the suspect.
"Anyone who was in the area of the underpass that runs beneath Wester Hailes Road towards Murrayburn around 9pm last night, who either witnessed the incident or who noticed the suspect there, should contact police.
"Likewise, anyone who recognises the description of the suspect, or who has any other information that can assist our inquiries, should also get in touch."
Police have asked that anyone with information about this incident contact them on 101 or Crimestoppers on 0800 555 111.