Phoebe Taylor, who is 17-years-old, was last seen by her family on Monday 27 August in Village Road, Whelpley Hill, Buckinghamshire.
Phoebe is described as around 5ft 5ins tall with red hair.
She was last seen wearing long colourful trousers with flowers and a white top.
She also has connections with Welwyn Garden City, Brighton, Chelmsford and London.
She is believed to have been left by friends in London in the Tottenham Court Road area, and was last spoken to on Wednesday 29 August.
Investigating officer Sergeant Darren Walsh of the Investigation Hub based at Amersham police station said: “We are growing increasingly concerned for the welfare of Phoebe, as it is completely out of character for her not to contact her family.
“I am appealing to anybody who may have seen her, or knows of her whereabouts to contact 101, quoting reference number 43180269535.
“Phoebe, if you see this, please call us to let us know that you are safe and well.
“You are not in any trouble, but we are concerned for your welfare and are asking you to make contact.”
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