Echinoecus pentagonus, the sea urchin crab, is a species of crab in the family Pilumnidae found from the Red Sea and East Africa to French Polynesia and the Hawaiian Islands.

[2] This crab is a parasite that lives in the rectum of a sea urchin.

In Hawaii, it chooses only Echinothrix calamaris, leaving few of these urchins unpopulated.

Its curved and pointed carapace reaches only 0.5 inches (1.3 cm) in width.

[3] Taxonomic synonyms of E. pentagonus include:[4]