Maria Gombosova, 14, of Westcliff, Essex, is thought to be with a man she might have met on the internet and could be in Leicester, police said.
The teenager is described as 5ft 7in (1.7m) tall with very long black hair, and was last seen wearing blue jeans, a red and orange coat and a pink shirt.
A police spokesman said the pair could be using a red Nissan car.
"Maria said she was popping out to see friends but never returned home," the spokesman added.
Police are growing increasingly concerned for the welfare of a girl missing from home since Saturday.