Starfish or starfish are asteroid like acanthodes. It is often found that these names also apply to serpentines, which are rightly called brittle stars or "basket stars.". From the tropics to the cold polar waters, there are about 1500 species of starfish on the seabed of all the oceans in the world. They extend from the intertidal zone to the depths of the seafloor, 6000 m (20000 ft) below the surface.

Starfish are marine invertebrates. They usually have a central disk and five arms, although some species have a large number of arms. The bottom or upper surface can be smooth, grainy or spiny and covered with overlapping plates. Many species are brightly colored in red or orange, while others are blue, gray or brown. Starfish's pins are operated by a hydraulic system with the mouth centered on the mouth or lower surface. They are opportunity feeders, mainly carnivores of benthic invertebrates. Some species have special feeding behaviors, including gastric valgus and suspended feeding. They have a complex life cycle and can reproduce sexually and asexually. Most weapons can regenerate damaged parts or lost weapons, and they can raise weapons as a defense. Asteroids play an important ecological role. Starfish, such as the Pisaster ochraceus and stichmaster australis, are widely regarded as examples of key species concepts in ecology. Acanthaster planci is a voracious predator of coral in the whole Indian Pacific region. The North Pacific starfish is considered to be one of the worst invasive species in the world.

The fossil record of starfish is ancient, dating back to the Ordovician period about 450 million years ago, but it is very poor because starfish tend to disintegrate after death. Only the small bones and spines of animals can be preserved, making it difficult to locate them. Starfish have attractive symmetrical shapes and play a role in literature, legend, design and popular culture. They are sometimes collected as antiques, designed or used as logos, and in some cultures can be eaten despite their potential toxicity.