Police say the incident happened in the Upgate/Mercer Row area last Wednesday (December 13), between 7pm and 8pm.
The victim is a woman in her 30s, who was approached from behind in the attack.
The offender was believed to be a man in his 30s who was tall, had short light-coloured hair, and stubble on his face.
He was wearing a tracksuit top which had coloured triangles on the sleeves with white borders around them.
The police are urging any potential witnesses to come forward, and have warned residents to be vigilant.
DS Scott Miller from Lincolnshire Police’s Emerald Team, said: “We are deeply distressed for the victim of this crime and our specialist officers are supporting her every step of the way through this investigation.
“We’re working on a number of lines of enquiry to try and establish the exact circumstances involved.
“If you were in this area on this evening and witnessed a struggle or something out of the ordinary please get in touch.
“If anyone will be in the area over the festive period please be extra vigilant and report any suspicious activity.”