Somto Igboayaka, 17, from East London, has been missing since February 11, police said.
Police said Somto 'frequents the Worthing area' but was last seen in Crawley.
Somto is described as black, 6ft 1 and of medium build.
A police spokesperson said: "There is increased concern for Somto's welfare and safety due to his age and the continued lack of information about him, and anyone who sees him or knows of his whereabouts is asked to contact Sussex Police on 101 or 999, quoting serial 1117 of 15/02."