Jayden Wallace, who is missing from Renfrewshire, is known to have links in the Aberdeen area.
Renfrewshire and Inverclyde police said the 15-year-old was last seen on a train at Langbank at 9.45pm on Tuesday, August 9.
The train was heading in the Glasgow direction.
Jayden is described as being 5ft 11in tall and of slim build. He has clean-shaven, blond hair.
He was last seen wearing a black Lacoste sweatshirt and trousers, black trainers and a black baseball cap.
The teenager is known to have links in the Aberdeen area and officers are asking anyone with information on his whereabouts to contact police.
This can be done using 101 and quoting incident number PS-20220809-3614.