Horseshoe crabs are marine and brackish water arthropods of the family Limulidae and the only living members of the order Xiphosura. Despite their name, they are not true crabs or crustaceans; they are chelicerates, most closely related to arachnids, such as spiders and scorpions.

Horseshoe crabs live primarily in and around shallow coastal waters on soft, sandy or muddy bottoms. They are generally found in the intertidal zone at spring high tides. They are eaten in some parts of Asia, and used as fishing bait, in fertilizer and in science (especially Limulus amebocyte lysate). In recent years, population declines have occurred as a consequence of coastal habitat destruction and overharvesting. Tetrodotoxin may be present in one horseshoe crab species, Carcinoscorpius rotundicauda.

Fossil records for horseshoe crabs extend back as far as 480 million years ago, with extant forms being living fossils. A 2019 molecular analysis places them as the sister group of Ricinulei within Arachnida.