Emily Besant, 15, was last seen shopping in Worthing on Saturday afternoon.
Sussex Police say she disappeared at around 2.40pm.
Sergeant Ian Maloney said: “We are concerned for Emily because of her age and she has now been missing over night. Although she has been living near Arundel she has links to Brighton and Crawley.
“She has also been known to travel to London.
“She is 5ft with very long, straight, dyed dark purple hair. She has her lip, tongue and belly button pierced. She was last seen wearing a short black jacket, a black and white scarf, white vest top and pale blue jeans.
“Anyone who has seen her or knows where she is, please contact on firstname.lastname@example.org quoting serial 825 of 19/03.”
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