The horseshoe crab or Atlantic horseshoe crab (Limulus polyphemus) is a marine chelicerate arthropod. Despite its name, it is more closely related to spiders, ticks, and scorpions than to crabs. Horseshoe crabs are most commonly found in the Gulf of Mexico and along the northern Atlantic coast of North America. A main area of annual migration is Delaware Bay, although stray individuals are occasionally found in Europe.
The other three species in the family Limulidae are also called horseshoe crabs. The Japanese horseshoe crab (Tachypleus tridentatus) is found in the Seto Inland Sea, and is considered an endangered species because of loss of habitat. Two other species occur along the east coast of India: Tachypleus gigas and Carcinoscorpius rotundicauda. All four are quite similar in form and behavior.
The extinct diminutive horseshoe crab, Lunataspis aurora, 4 centimetres (1.6 in) from head to tail-tip, has been identified in 445-million-year-old Ordovician strata in Manitoba.