Robert McKenzie went missing from Gourock on Thursday.
He is described as 5ft 2in with a slim build, short sandy coloured hair and blue eyes.
He was wearing a blue Adidas tracksuit and white Adidas trainers.
Police officers in Greenock yesterday took to social networks in a bid to trace him.
A spokeswoman for Police Scotland said: "Police are concerned for the welfare of 15-year-old Robert McKenzie who has been missing from his home address in Gourock since 6.50pm on Thursday April 10, 2014."
Contact Greenock Police on 101.