Officers from Northamptonshire Police say they are growing concerned for the safety of 17-year-old Amy Pitt, who has not been seen since the beginning of November.
Amy was last seen in Daventry and is now believed to be in the Northampton area.
She has blue eyes and dark-brown hair which she tends to wear in a ponytail.
When she was last seen she was wearing black denim jeans, black denim trainers and a black hooded sweater.
Police are urging Amy, or anyone who knows where she might be, to contact them on 101.