Jasmine Coleman, of Lancing, West Sussex, hasn't been seen since the early hours of today.
She was seen at 11.30pm in her bedroom, but was gone by 2am.
A spokesperson for Sussex police said: "It is thought Jasmine may have gone off in a car with an older man, travelling towards London or the A24, or may have travelled somewhere by train.
"Sussex Police officer are working with colleagues from Surrey Police, British Transport Police and the Metropolitan Police in an urgent effort to trace Jasmine."
Jasmine is described as white, slim, with blonde hair and blue eyes. She is also said to look older than her age.
She could either be wearing a paisley print dress or a polo neck with black riding trousers.
Police said their inquiries have lead them to the Croyden area.
Anybody with information about Jasmine's whereabouts are urged to contact Sussex Police.
You can email 101@sussex.pnn.police.uk or call 101 and quote serial 145 of 21/3.
[Images/Sussex Police, Alamy]