Sussex Police said it is 'increasingly concerned' for the welfare of 16-year-old Klodian Marku, who has been missing from his home in Brighton for several days.
He was last seen at around midday on Sunday (May 21), police said.
"This is extremely out of character for Klodian and he has never failed to return home previously," a police spokeperson said.
Klodian is described as white, 5' 10" and of medium build with black hair. He was last seen wearing a black puffa style jacket, grey tracksuit bottoms and a grey T-shirt.
Police said Klodian may be using the local buses to travel around the city.
If anyone sees Klodian or knows of his whereabouts, call police on 101 quoting serial 1472 of 21/05. If he is in danger or needs urgent medical attention dial 999.