Sussex Police is concerned for the welfare of 15-year-old Stephanie McGuinness.
Stephanie was last seen in Brighton on Friday, December 6 and is believed to have travelled by train to the Stratford area of London.
She is white, of a slim build, 5ft4in and has very long dark brown and blonde hair.
Anyone who has seen Stephanie or has information on her whereabouts is urged to report online or call 101, quoting serial 308 of 07/12.