The offence took place near a ginnel off Stockwell Road between 3.35pm and 3.43pm on yesterday as the girl was walking home from school.
She ran home and informed her parents about what had happened and they reported it to the policethat evening.
Police enquiries have taken place in the neighbourhood, including a trawl through CCTV and speaking to residents.
There were potential witnesses nearby at the time, including a man on a mobility scooter. They have been urged to contact the police.
Officers are working with schools after the offence and patrols are also being carried out in the area while the investigation continues.
The culprit is described as being 6ft tall, of slim build, with short dark and grey hair, and he was wearing a checked top and baggy blue jeans.
Anyone with information is asked to call 101, slect option one, and speak to the Force Control Room.
Information can also be passed to Crimestoppers anonymously on 0800 555 111.
Quote reference number 12180048511 when providing details.