The girl was grabbed from behind by a man who attempted to touch her inappropriately, officers say.
The incident happened between 9pm and 10pm on June 6, although it was only reported to police on July 8.
Between 9pm and 10pm on June 6th a man approached a girl, aged 16, and grabbed her from behind before attempting to touch her inappropriately.
The incident was reported to police on July 8.
The victim, who was with her friend at the time, was wearing a distinctive orange crop top and white shorts and her female friend was wearing a denim jacket.
The suspect is described at 5ft 8in and was wearing red shorts, a plain white t-shirt and trainers. He was with five other men at the time who appeared to be laughing.
Detective Constable Daniel Walker said: “We take all reports of sexual assault seriously, no matter when it is reported.
“There are various reasons which may lead to a delay in the victim reporting sexual assaults to us and we would ask the public to be mindful of this.
“We believe this to be an isolated incident and we would encourage anyone who may have any information to contact us.
“Any information, no matter how irrelevant you may feel it is, could help us to identify this suspect.”
If you were in the area at around this time or have any information that can assist with the police investigation, call 101 and quote reference 21000380794
Police would also like to hear from anyone who may have CCTV of the area on this day.
Lincolnshire Police say it is important that anyone who has been a victim of rape or sexual assault remembers that what has happened is not their fault and there is help available.
A spokesperson for the force said: "If you are hurt and need medical attention, call 999 and ask for an ambulance or go to your local Accident and Emergency department.
"Try not to be alone. See if a friend or someone you trust can be with you. Talk to someone about what has happened. In addition to this, or if you don't feel like talking to a friend or family member yet, please consider contacting the sexual assault referral clinic Spring Lodge or Lincolnshire Rape Crisis.
"To report any recent or non-recent incidents to us call 101 or 999 if it’s an emergency. We have specially trained Officers to investigate and support you through the process."