The victim, aged 17, was on King’s Road walking towards Coppice Drive on Monday afternoon when the man approached her from behind and touched her.
North Yorkshire Police said the man, thought to be in his twenties, touched the girl on her bum and chest.
He then walked back in the direction he had come from towards the A61 junction following the assault, which occurred at around 3.50pm.
The suspect is described as white, aged in his twenties, thin build, long face and clean shaven, and he has long, light-brown and slightly curly hair which was tied back. He was wearing a red and grey backpack.
The victim and her family are being supported by the police while enquiries continue.