The victim, 30, was attacked by the man around midnight Saturday October 5 into Sunday October 6.
The assault took place in a lane near to Commercial Street in the Shetland capital, with police undertaking extensive inquiries in the area.
Additional officers have also been deployed in Lerwick to trace the suspect, who police described as being in his late 20s to early 30s, between 5ft 8in and 6ft in height, with dark hair and a slim build.
Detective Inspector Gerry Shovlin said: ."This is an extremely unusual incident in Lerwick and I would like to reassure people that considerable efforts are being made to progress inquiries as swiftly as possible
"I would urge anyone who was around the Commercial Street area between 10pm and 1.20am on the night in question to come forward.
"You may have information which could help so please cast your mind back and pass on anything which may be of use. I would also be interested in speaking to anyone who was driving in the area and may have dashcam footage.
"Anyone who can help can contact police on 101, quoting incident 0090 of 6 October, or call Crimestoppers anonymously on 0800 555 111."