Growing to a maximum diameter of 26 cm (10 in), Echinaster callosus has a small central disc and five slender cylindrical arms. The aboral (upper) surface is densely covered by warty protuberances. The colour of this starfish is variable, but may be orange, pink or violet, with the warts being a contrasting colour, usually white, pink, red or mauve. The white warts often occur in bands, particularly towards the tips of the arms. There are small yellowish-green retractable pedicellaria between the warts. The oral (under) surface of the starfish is white with white warts. There are ambulacral grooves on the underside of the arms along which food is passed by ciliary action. The tip of each arm bears an eyespot, as well as a bundle of suckers.