Grant Maloney was last seen at 4pm on Friday, July 29 when he left his high school at Elizabeth.
Police said he has not made contact with anyone or been seen since this date, and that they are concerned for his welfare.
He was wearing a grey and red striped hooded jumper and grey tracksuit pants at the time.
Police have received information Grant could be in Darwin.
He is described as Caucasian with a fair complexion, 160cm, about 45kg, slim build with collar-length light brown/blonde scraggly hair.
Anyone with information should contact the Police Attendance Line on 131 444 or BankSA Crime Stoppers on 1800 333 000.