The force says a suspect allegedly fled the scene barefoot, topless and carrying a white T-shirt which he is thought to have possibly put on later.
He is described as being in his late teens or early 20s, around 5ft7, having a six-inch tattoo on his back between his shoulder blades, short 'styled' brown hair and a stubbly moustache. He allegedly left behind a green UCLA hoodie as well as some 'well-worn' Adidas trainers.
The complainant is being supported by Dewberry House Sexual Assault Referral Centre.
Anyone with information or those who may have seen the suspect, particularly those who were in the Maufant area of St Martin between 3am and 5am on Saturday night, is asked to contact police on 612612.
Any footage of the area – recorded on dash cams or CCTV between the specified times – is also being requested by the force.
Detective Chief Inspector Craig Jackson, who is overseeing the investigation, said: 'I would urge either the individual or anyone who recognises the description and clothing to make contact with police as soon as possible.'
Information can also be given anonymously by calling Crimestoppers on 0800 555111 or by going on their website.