Megan Cross was last seen at her home in Chorley at around 10.30am Tuesday, 17 January.
She is described as white, 5ft tall, with long light brown hair. She was last seen wearing a royal blue coat with a white fur hood, black leggings with orange Ellesse logos and black trainers.
Police believe she could be in the Morecambe or Blackpool areas.
A police spokesperson said: "If anyone has seen Megan or knows where she is we would urge them to contact us as soon as possible."
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Anyone with information can contact police on 101 quoting log 1482 of 17th January.