Elena Dimitru, 15, left the home in Spalding around 10.30pm on Monday July 7 and has not made contact since.
She is described as a "streetwise" but had only lived in Lincolnshire for a week and is not familiar with the area.
Elena is 5ft 5in tall, of medium build, with very long dark hair. She has a tattoo on her left hand saying 'Ionut'.
She was wearing a light pink zip-up hoody, dark leggings and black plimsolls when she was last seen.
Anyone with information regarding her current whereabouts is asked to contact PC Ryan Crombie at Spalding Police Station, by calling 101 and quoting the incident number.