South Yorkshire Police said the girl had been on Brunswick Road in the Pitsmoor area of Sheffield when she was assaulted on Monday lunchtime
Officers said the teenage attacker had covered part of his face with a cloth.
He is described as having a local accent, of slim build, and wearing a dark-hooded shell suit jacket and dark bottoms.
The girl's mobile phone was stolen by the youth after the attack.
A 14-year-old boy arrested in connection with the attack was later released without charge.
Det Insp Dave Mayfield said: "The victim was left traumatised by her ordeal, and is now recovering at home with her family.
"We are treating this as an isolated incident, however I would warn people to be vigilant and careful when out alone."
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