16 year old Mia Copsey went missing from her home in Watford on Monday 19 August.
She has been seen in the past few days in the area between Sewardstone Close, Waltham Abbey and Chingford.
Officers are growing increasingly concerned for her welfare.
Mia is described as 5ft 7in tall, of a slim build and with medium length straight brown hair.
If you have seen Mia since she went missing or have information on her whereabouts, please call the non-emergency number 101. For immediate sightings, call 999 straight away.