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. 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 in width.
Taxonomic synonyms of E. pentagonus include: Echinoecus klunzingeri Miyake, 1939 Echinoecus pentagonus Rathbun, 1894 Echinoecus rathbunae Miyake, 1939 Eumedon convictor Bouvier & Seurat, 1906 Eumedon pentagonus A. Milne-Edwards, 1879 Eumedonus petiti Gravier, 1922 Liomedon pentagonus Klunzinger, 1906
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