A teenager is thought to have travelled 40 miles by himself to Brighton.
Mason King, 13, was reported missing from his home in East Beach Road, Selsey, on Sunday morning.
He was spotted twice in Brighton that evening and Sussex Police believe that he is probably still in the city.
The teenager has called home to say he is OK but officers say due to his age there are real concerns for his welfare.
The youngster is white, 5ft 6in, and slight with short darkish blonde hair. He was wearing a blue and grey hooded top, blue jeans and black and white Nike trainers when he disappeared.
Email information to firstname.lastname@example.org or call 101 quoting serial 677 of 31/08.