This sea star has five stout rays that range in length from 4 to 10 inches. The rays are arranged around an ill-defined central disk. While most of them are purple, they can be orange, orange-ochre, yellow, reddish, or brown. The aboral surface contains many small spines called ossicles that are arranged in a netlike or pentagonal pattern on the central disk. The Pisaster has tube feet that have suckers on their distal ends which allow them to attach to the rocky substrate and live in heavily wave-swept areas.
Pachygrapsus crassipes can be red, purple, or green. The back shell is a boxy shape, it is more broader than it is long. A distinctive feature is the series of horizontal lines across the carapace. In this species, the males are larger than the females. The size difference is noticeable after the crab's carapace reaches the width of 22 mm. The female's carapace becomes narrower and shorter than that of a male. The difference of the brachyuran on the abdomen is apparent.
Pachygrapsus crassipes can reach the size of 47.8 mm for males and females the carapace can reach the size of 40.8 mm
It is native to the coastline of western North America, where it is a common kelp of the intertidal zone. It is dark brown in color, shiny and bumpy in texture, and may reach over five meters long. It grows a branching stipe from a thick holdfast. It bears long, flat, straplike fronds lined with small blades each a few centimeters long.