Ceramaster patagonicus is pentagonal in shape and up to 8.2 cm in greatest radius, often with the aboral surface swollen and soft to the touch in living specimens. It is creamy orange to red-orange on the aboral side and pale yellow orally. The ratio of arm to disc ranges from 1.3 to 1.7. C. patagonicus has regular hexagonal aboral plates on the radii of the arms, and square or rhomboid plates between the radii. Each aboral tabulate plate is covered with 12 to 15 marginal granules and 4 to 12 central granules; a few plates bear spatulate pedicellariae. The marginals are massive and the granules crowded together. The oral intermediates are four sided, with granules and pedicellariae similar to those on the aboral plates. The adambulacrals have 3 to 5 robust spinelets on the edge of the furrow; distal to these on the oral surface is a longitudinal row of 2 or 3 short, stubby spinelets and then 5 to 8 irregular granules. The mouth plates have 8 or 9 blunt, prismatic marginal spines.
Similar SpeciesCeramaster patagonicus is larger than C. arcticus, and it has more granules on the aboral tabulate plates and more furrow spines.