Amandla Ndlovu was seen near the Odeon cinema on Putney High Street at 5.30pm on Friday evening.
She is described as a black girl of small build, 5ft 1in tall wearing blue jeans, a white jumper and a black coat.
She was carrying a black rucksack.
It is thought she is in the company of an older friend, who is not a missing person, and that since Friday they have been in the Oxford Circus, Croydon town centre and Brixton town centre areas.
Miss Ndlovu does not have a mobile phone, and police officers have been unsuccessful in attempts to contact her friend.
It is also thought she does not have an Oyster card.
Met Police detectives are concerned for her safety and she is considered a vulnerable person.
Anyone with information about her whereabouts should contact Lambeth police on 101 and quote 15MIS050664.