The 37-year-old was assaulted at a bus stop on Calder Road, in Edinburgh, some time between 12.50am and 1.15am on Friday.
The bus stop is located between the junction at Calder Crescent and the underpass near to Wester Hailes Road.
Detective Constable Joyce Gunderson said: "This was a serious incident in the very early hours of the morning but we believe that some members of the public may have seen something suspicious in and around Calder Road, and we urge those people to get in touch immediately.
"The suspect is male and it is understood that he was wearing dark clothing at the time. Anyone with any information should contact Police Scotland on 101 or Crimestoppers on 0800 555 111."