The teenager was approached by the man at about 11.20pm on Wednesday, on South Road, near its junction with Arran Drive, in the Charleston area of Dundee.
He sexually assaulted her before running off along towards Menzieshill. The girl was uninjured but extremely distressed, Tayside Police said.
The man is described as smelly, in his 20s, 5ft 10ins, with a stocky build and a Dundee accent. He wore a black hooded top, black trousers and black shoes.