Scott Armstead, 13, was reported missing from his home in Horsforth at about 10pm yesterday (Tuesday).
Officers now say they are concerned for his welfare and have issued an urgent appeal for his whereabouts.
He is described as being white, about 6ft 1in tall, with ginger hair.
Scott was last seen wearing a khaki Nike tracksuit, khaki Nike trainers and he was carrying a khaki Nike man-bag.
Anyone who has seen him or who has any information that could assist in tracing him is asked to contact officer via 101, quoting log number 2039 of August 28.