The victim, described by detectives as “in her very early teens”, was walking towards a duck pond in Chislehurst when she was grabbed from behind by a stranger.
The man dragged her towards some isolated woods where he sexually assaulted her for about 30 minutes before fleeing.
The girl went home and reported the attack, which happened at about 3.30pm on Friday April 28 between Ashfield Lane and Heathfield Lane.
Police today released an e-fit of the suspect using a description provided by the victim who officers praised as “extraordinarily brave”.
Detective Sergeant Ross McEwen, from the Met’s sexual offences and child abuse squad, said: “This incident occurred in the late afternoon in an area popular with walkers.
“There would have been people in the woods and surrounding areas and we are keen to speak to them.
“It does not matter whether you think you saw something significant or not, your recollections of that day may just provide us with the vital information required to identify the person responsible for subjecting such a young victim to a horrendous attack.”
He added: “The victim has been extraordinarily brave and has worked with us to produce the e-fit. If anyone recognises this man or has any information I would urge them to contact us immediately as this man needs to be identified as soon as possible.”
The attacker was described as a white man, 5ft 5ins tall, of medium build with a grey beard. He was wearing blue jeans, a blue top, a blue body warmer, a grey woollen hat and slip-on plimsoll shoes.
Any who has any information is urged asked to contact detectives on 020 8721 4610 or by tweeting @MetCC.