Mohammed Umar Khan, 16, was last seen in Dewsbury around 4.30pm on Monday and was reported missing later that night.
It is thought he travelled to London on Monday night.
He is described as 5ft 8ins, slim, with short, black hair which is shaven at the sides, and brown eyes.
He was last seen wearing a green Forbidden winter coat with fluffy brown hood, black satin Adidas jogging bottoms with stripes, green Nike Airmax shoes and was carrying a blue backpack.
Anyone who has seen him, or has information about his whereabouts is asked to contact West Yorkshire Police on 101.