The boy was found with a stab wound outside The Railway pub in North Road, Lancing last night.
Officers were called to the scene about 11.05pm and found treating the victim when they arrived.
He was taken to hospital with non-life-threatening injuries which required stitches and has since been discharged.
Detective constable Lee Simmonds said: “We would like to hear from anyone who was in the area at the time and who saw something suspicious.”
Witnesses can email [email protected] or call 101 quoting reference 1760 of 01/09.
Alternatively members of the public can call the independent charity Crimestoppers anonymously on 0800 555 111 or visit www.crimestoppers-uk.org.
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