The victim was walking in Robbiesland Drive towards Holmhead Road in Cumnock, East Ayrshire, at around 10.30pm on Sunday when she was approached by a man wearing a balaclava who pushed her to the ground.
Police said he then "touched her inappropriately over her clothing" before running off.
The 15-year-old was left upset and with a graze to her hip, but she did not require hospital treatment.
She managed to flag down a passing car after the assault and the driver took her to a police station.
The suspect is white, around 5ft 7in, well built, and was wearing dark tracksuit bottoms, a navy top with white diagonal writing on the chest and a balaclava.
Residents living in the street heard screams around the time of the attack and police are appealing for any witnesses to contact them.
Detective Constable Sarah Cawley said: "Although unhurt, this was a very distressing incident for the young girl. She managed to flag down a car after the assault and they kindly took her to the local police office.
"We know from our house-to-house in the area that a couple of the local people did hear screams at that time, so I would like to hear from anyone who may have seen or heard something that may help us catch the man responsible.
"Information can be passed to the CID at Kilmarnock police office on the non-emergency number 101 or in confidence to Crimestoppers on 0800 555 111."