Scott Diver, 16, was last seen at around 2.45pm on Saturday September 17 at his home in Second Avenue in Clydebank.
He was captured on CCTV walking through nearby Dalmuir Park at around 2.53pm the same day.
He is described as around 5ft 7in tall, of slim build, with blue eyes and dark hair. He was last seen wearing a navy Adidas tracksuit with light blue stripes down the arms and legs, a white T-shirt and navy Adidas trainers with grey stripes.
Inspector John Mullen said: “It has now been three weeks since Scott was reported missing and his family are extremely worried about him.
“I would continue to urge anyone who recognises Scott, or who has any information regarding his whereabouts, to contact officers as soon as possible.
“We know that Scott was captured on CCTV walking through Dalmuir Park, in the direction of the park entrance at Mountblow Road, around 2.53pm on the day he went missing. I would ask anyone who may have seen him that day to come forward as you may have information that could assist with our inquiry.
“I would also appeal directly to Scott to please get in touch with someone to let us know he is safe. You’re not in any trouble, but there are a lot of people worried about you and we need to know that you are okay.”