Brooke Nicholas, 16, was last in the St. Thomas Road area of Preston at approximately 3.15pm on May 19.
She is described as being a white female, 5ft 3in tall, of slim build with short brown hair.
Brooke is believed to be wearing a coral-coloured Nike hoodie, green North Face jacket, black North Face leggings and black Nike trainers.
She has links to Preston and Blackpool.
A spokesperson for Lancashire Police said: "We're appealing for information in relation to the whereabouts of Brooke Nicholas, 16-years-old from Preston, who has been reported missing."
If you have information contact police by calling 101, quoting log reference number 04/80757/20.