Abstract: A protective retainer for fastening and retaining the side wall of a protective liner in a pickup truck cargo bed is disclosed which is particularly constructed to maintain the engaged liner side wall in substantial longitudinal alignment despite varying environmental conditions. The protective retainer has a fastening plate which extends longitudinally to substantially cover the side surface of the side rail of a truck cargo bed. A retainer section depends downwardly from the fastening plate. The retainer section includes inner and outer downwardly depending walls which closely engage and retain the side wall of a truck bed liner. The retainer section walls permit vertical and longitudinal movement of the liner side wall within the retainer section, but substantially prevent transverse displacement of the engaged portion of the liner side wall.