Clypeasteroida subdivision_ranks = Suborders and families subdivision = See text.

Sand dollar, or Sand-dollar is a name used for many species of flattened, burrowing sea urchins belonging to the order (Order (biology)) Clypeasteroida. The Echinarachnius parma, also known as the Common Sand Dollar, is widespread in ocean waters of the Northern Hemisphere, from the intertidal zone to considerable depths. It can be found in temperate and tropical zones.

All members of Clypeasteroida have a rigid skeleton known as a test (test (biology)). This is the typical fossil found washed up on beaches. The living animals have a skin of motile spines covering the test. Movement is accomplished by the coordinated action of the spines. Like other sea urchins, sand dollars have five paired rows of pores. The pores are arranged in a petal-like pattern. These pores are perforations in the endoskeleton through which the podia, used in gas exchange, project from the body.