We present an anatomical redescription and reassessment of the phylogenetic relationships of Sclerosaurus armatus Meyer in Fischer, 1857, a parareptile from the Lower to lower Middle Triassic Buntsandstein of southern Germany and northern Switzerland. This taxon is distinguished from other known parareptiles by the possession of long, posterolaterally projecting spikes formed by supratemporals, posterior dentary teeth with mesiolingually aligned and slightly imbricating crowns, and a narrow band of dorsal dermal armor comprising two or three rows of sculptured osteoderms on either side of the midline. Phylogenetic analysis places Sclerosaurus in the Procolophonidae rather than as the sister taxon of Pareiasauria. Furthermore, certain apomorphic character states, particularly the presence of three spines on the quadratojugal, support its referral to the Leptopleuroninae. Sclerosaurus resembles pareiasaurs in the presence of an intercondylar canal on the distal end of the femur, the absence of gastralia, and having a short tail. It is possibly the largest procolophonid known to date.