A man approached the 16-year-old, making inappropriate comments before he assaulted her.
Police are now appealing for anyone with information to come forward.
The incident took place in the Station Road area around 5.40pm on January 27.
The suspect is described as white, around 28 years old, with a shaved head, green eyes, and wearing a black jacket – possibly Nike or Adidas, a light-coloured t-shirt and black trousers.
He was last seen heading towards Polmont train station.
Detective Sergeant Donald Rodger of Falkirk CID said: “We’ve been conducting extensive enquiries in the area since this assault was reported to us and we’re now asking for the help of the community.
“Anyone who may have seen this man in the area, or who recognises his description, is urged to contact either Falkirk CID or the charity Crimestoppers in anonymity as soon as possible.”
Those with information can contact Police Scotland on 101, quoting incident number 3107 of January 27 or report this anonymously to Crimestoppers on 0800 555 111.