Horseshoe crabs are arthropods of the family Limulidae and the only surviving xiphosurans. Despite their name, they are not crabs or even crustaceans; they are chelicerates, more closely related to arachnids like spiders, ticks, and scorpions. The body of a horseshoe crab is divided into three main parts: the cephalothorax, abdomen, and telson. The largest of these, the cephalothorax, houses most of the animal's eyes, limbs, and internal organs. It is also where the animal gets its name, as its shape somewhat resembles that of a horseshoe. Horseshoe crabs have been described as "living fossils", having changed little since they first appeared in the Triassic.

Only four species of horseshoe crab are extant today. Most are marine, though the mangrove horseshoe crab is often found in brackish water, and the Atlantic horseshoe crab is resident in brackish estuarine ecosystems such as the Delaware and Chesapeake bays. Additionally, certain extinct species transitioned to living solely in freshwater. Horseshoe crabs primarily live at the water's bottom but they can swim if needed. In the modern day, their distribution is limited, only found along the coasts of the western Atlantic Ocean in North America, and the Central Indo-Pacific in South and Southeast Asia.

Horseshoe crabs are often caught for their blood, which contains Limulus amebocyte lysate, a chemical used to detect bacterial endotoxins. Additionally, the animals are used as fishing bait in the United States and eaten as a delicacy in some parts of Asia. In recent years, horseshoe crabs have experienced a population decline. This is mainly due to coastal habitat destruction and overharvesting. To ensure their continued existence, many areas have enacted regulations on harvesting and established captive breeding programs.

The fossil record of xiphosurans extends back to the Late Ordovician, or around 445 million years ago. For modern horseshoe crabs, their earliest appearance was approximately 250 million years ago during the Early Triassic. Because they have seen little morphological change since then, extant (surviving) forms have been described as "living fossils".

Horseshoe crabs resemble crustaceans but belong to a separate subphylum of the arthropods, Chelicerata. Horseshoe crabs are closely related to the extinct eurypterids (sea scorpions), which include some of the largest arthropods ever to have existed, and the two may be sister groups. The difficult-to-classify chasmataspidids are also thought to be closely related to horseshoe crabs.

The radiation of horseshoe crabs resulted in 22 known species, of which only 4 remain. The Atlantic species is sister to the three Asian species, the latter of which are likely the result of two divergences relatively close in time. The last common ancestor of the four extant species is estimated to have lived about 135 million years ago in the Cretaceous.

Limulidae is the only extant family of the order Xiphosura, and contains all four living species of horseshoe crabs:

After Bicknell et al. 2021 and Lamsdell et al. 2020