The 15-year-old was in Glasgow Street in Ardrossan, Ayrshire, when she was approached by a man at about 2am on Saturday.
He is said to have engaged her in conversation and they went into a lane or a close just off the road, where she was assaulted.
The man is described as being about 18, 6ft, with a thin build and blonde/brown hair. He was wearing a blue top with "Henley" on the front and blue jeans.
The girl was later assisted by police on patrol in the area.
Detective Inspector William McDicken, of Saltcoats CID, appealed to anyone in the area around the time of the attack to contact him.