Agonus cataphractus has a wide, flattened, triangular head (around 3.5 times the body length) with an elongated, tapering body. Instead of scales this fish is completely covered in hard bony plates, that form lateral rows of sharp spines. This fish typically grows to 10-15 cm but has been recorded at a length of 21 cm. The upper parts of the body are greyish-brown in colour with 4-5 darker saddles across the back, and the lower parts are lighter sometimes with grey spots. The fins are yellow with dark stripes and spots, and the underside of the fish is a creamy white colour. There are two pectoral fins located close together; the first with 5-6 spines and the second with 6-8 soft fin rays, and an anal fin, which is short with 6-7 fin rays. The snout has a pair of strong, sharp spines, and a very sharp spine on each gill cover. The underside of the head has many small barbels extending from the tip of the snout to the edge of the gill covers. The mouth is located beneath the head.The pectoral fins can have an orange tint in the breeding season. This species is also commonly known as the 'hook-nose' or 'armed bullhead'.