Ellie Tennant was last spotted at around 4.30pm yesterday where she was believed to have been boarding a train from South Woodham Ferrers to Southend with a middle-aged woman with a white Staffordshire dog.
She is also believed to be wearing a bottle-green school uniform.
A police spokesman said: "She is believed to be with people known to her in the Southend area."
The person she is with is not believed to be dangerous.
Her family are pleading for her safe return.
Anyone with information or who has seen Ellie should contact Essex Police on 101.