Caine O'Brien, 17, was last seen on July 28, though police say it is believed he was a passenger in a car that was stop-checked in the Ruabon area of north Wales on Monday.
He is described as a white male, 5ft 11in tall, of medium build with collar-length curly brown hair.
When he was last seen, Caine, who lives in St Helens, was wearing a blue coat with a hood and blue tracksuit bottoms. He was carrying a black bag.
Anyone who has seen Caine or knows of his whereabouts is asked to contact Merseyside Police on Twitter @MerPolCC or call them on 101.
Any sightings of him can also be reported online here