Parastichopus californicus can measure up to 50 cm long; it is the largest of the sea cucumbers in British Columbia waters. It is cylindrical with slightly tapered ends. The dorsal side has about 40 large, and many smaller, flesh-coloured papillae. The skin colour varies from mottled brown to a more solid brown or red in juveniles; all-white specimens occur rarely. The ventral side of the body has numerous rows of robust tube feet, and is usually lighter than the dorsal side. A circle of 20 peltate feeding tentacles surrounds the subterminal mouth (at the end but pointing down).
Skin ossicles: tables with a disc diameter of 72 to 92 μm and a spire with 11 to 19 spines; large oval plates with two rows of holes running lengthwise.
Similar SpeciesParastichopus californicus might be confused with P. leukothele. In life, P. leukothele is quite distinct with bright orange skin and rusty-brown patches. The papillae are small and white. It also lives at greater depths, and only occasionally would be seen by scuba divers. In a preserved specimen, the differences are less obvious, and one needs to make an ossicle preparation and count the number of spines on the table.
Synallactes challengeri is also similar in appearance, but it is smaller and differs in colour and in the length of the dorsal papillae. It is usually grey with a purple or pinkish tinge, and the papillae are longer and more slender.