Starfish or sea stars are echinoderms belonging to the class Asteroidea. The names "starfish" and "sea star" essentially refer to members of this class. However, common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as "brittle stars" or "basket stars". About 1,500 living species of starfish occur on the seabed in all the world's oceans, from the tropics to subzero polar waters. They are found from the intertidal zone down to abyssal depths, 6,000 m (20,000 ft) below the surface.

Starfish are among the most familiar of marine invertebrates. They typically have a central disc and five arms, though some species have more than this. The aboral or upper surface may be smooth, granular or spiny, and is covered with overlapping plates. Many species are brightly coloured in various shades of red or orange, while others are blue, grey or brown. Starfish have tube feet operated by a hydraulic system and a mouth at the centre of the oral or lower surface. They are opportunistic feeders and are mostly predators on benthic invertebrates. Several species having specialized feeding behaviours including eversion of their stomachs and suspension feeding

They have complex life cycles and can reproduce both sexually and asexually. Most can regenerate damaged parts or lost arms and they can shed arms as a means of defence. The Asteroidea occupy several significant ecological roles. Starfish, such as the ochre sea star (Pisaster ochraceus) and the reef sea star (Stichaster australis), have become widely known as examples of the keystone species concept in ecology. The tropical crown-of-thorns starfish (Acanthaster planci) is a voracious predator of coral throughout the Indo-Pacific region, and the northern Pacific sea star is considered to be one of the world's 100 worst invasive species.

 The fossil record for starfish is ancient, dating back to the Ordovician around 450 million years ago, but it is rather poor, as starfish tend to disintegrate after death. Only the ossicles and spines of the animal are likely to be preserved, making remains hard to locate. With their appealing symmetrical shape, starfish have played a part in literature, legend, design and popular culture. They are sometimes collected as curios, used in design or as logos, and in some cultures, despite possible toxicity, they are eaten.

The scientific name Asteroidea was given to starfish by the French zoologist de Blainville in 1830. It is derived from the Greek aster, ἀστήρ (a star} and the Greek eidos, εἶδος (form, likeness, appearance). The class Asteroidea belongs to the phylum Echinodermata. As well as the starfish, the echinoderms include sea urchins, sand dollars, brittle and basket stars, sea cucumbers and crinoids. The larvae of echinoderms have bilateral symmetry but this is lost during metamorphosis after which they develop radial symmetry, typically pentamerism. Adult echinoderms characterized by having a water vascular system with external tube feet and a calcareous endoskeleton consisting of ossicles connected by a mesh of collagen fibres

The starfish are included in the subphylum Asterozoa, the characteristics of which include a flattened, star-shaped body as adults consisting of a central disc and multiple radiating arms. The subphylum includes the two classes of Asteroidea, the starfish, and Ophiuroidea, the brittle stars and basket stars. The asteroids have broad-based arms with skeletal support provided by calcareous plates in the body wall while ophiuroids have clearly demarcated slender arms strengthened by paired fused ossicles forming jointed "vertebrae"

 The starfish are a large and diverse class with about 1,500 living species. There are seven extant orders, Brisingida, Forcipulatida, Notomyotida, Paxillosida, Spinulosida, Valvatida and Velatida and two extinct ones, Calliasterellidae and Trichasteropsida .

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