Mary-Kate Morrison, aged 16 and from Preston, was last seen in person in Blackpool on Friday, April 26 but was captured on CCTV at Sheffield railway station at around 4.45pm the following day.
Officers believe the teenager could be in Sheffield or Barnsley.
She is white, around 5ft 4ins tall, slim and has very long, mid-brown hair in an ombre style, which is often tied back in a ponytail.
She was last seen wearing blue jeans, white trainers and a light coat.
Anybody with information on her whereabouts should call Lancashire Constabulary on 101 and quote log number 27 of April 27.