A squid (pl. squid) is a mollusc with an elongated soft body, large eyes, eight arms, and two tentacles in the orders Myopsida, Oegopsida, and Bathyteuthida (though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fitting these criteria). Like all other cephalopods, squid have a distinct head, bilateral symmetry, and a mantle. They are mainly soft-bodied, like octopuses, but have a small internal skeleton in the form of a rod-like gladius or pen, made of chitin.

Squid diverged from other cephalopods during the Jurassic and radiated at the beginning of the Late Cretaceous, and occupy a similar role to teleost fish as open-water predators of similar size and behaviour. They play an important role in the open-water food web. The two long tentacles are used to grab prey and the eight arms to hold and control it. The beak then cuts the food into suitable size chunks for swallowing. Squid are rapid swimmers, moving by jet propulsion, and largely locate their prey by sight. They are among the most intelligent of invertebrates, with groups of Humboldt squid having been observed hunting cooperatively. They are preyed on by sharks, other fish, sea birds, seals and cetaceans, particularly sperm whales.

Squid can change colour for camouflage and signalling. Some species are bioluminescent, using their light for counter-illumination camouflage, while many species can eject a cloud of ink to distract predators.

Squid are used for human consumption with commercial fisheries in Japan, the Mediterranean, the southwestern Atlantic, the eastern Pacific and elsewhere. They are used in cuisines around the world, often known as "calamari". Squid have featured in literature since classical times, especially in tales of giant squid and sea monsters.

Squid are members of the class Cephalopoda, subclass Coleoidea. The squid orders Myopsida and Oegopsida are in the superorder Decapodiformes (from the Greek for "ten-legged"). Two other orders of decapodiform cephalopods are also called squid, although they are taxonomically distinct from squids and differ recognizably in their gross anatomical features. They are the bobtail squid of order Sepiolida and the ram's horn squid of the monotypic order Spirulida. The vampire squid (Vampyroteuthis infernalis), however, is more closely related to the octopus than to any squid.

The cladogram, not fully resolved, is based on Sanchez et al., 2018. Their molecular phylogeny used mitochondrial and nuclear DNA marker sequences; they comment that a robust phylogeny "has proven very challenging to obtain". If it is accepted that Sepiidae cuttlefish are a kind of squid, then the squids, excluding the vampire squid, form a clade as illustrated. Orders are shown in boldface; all the families not included in those orders are in the paraphyletic order "Oegopsida", except Sepiadariidae and Sepiidae that are in the polyphyletic order "Sepiida",

Crown coleoids (the common ancestor of octopuses and squid) diverged in the late Paleozoic (Mississippian), according to fossils of Syllipsimopodi, an early relative of vampire squids and octopuses. True squid diverged from other cephalopods during the Jurassic, but only emerged to prominence at the boundary between the Early and Late Cretaceous, around 100 million years ago. Both the coleoids and the teleost fish were involved in much adaptive radiation at this time, and the two modern groups resemble each other in size, ecology, habitat, morphology and behaviour, however some fish moved into fresh water while the coleoids remained in marine environments.

The ancestral coleoid was probably nautiloid-like with a straight septate shell that became immersed in the mantle and was used for buoyancy control. Four lines diverged from this, Spirulida (with one living member), the cuttlefishes, the squids and the octopuses. Squid have differentiated from the ancestral mollusc such that the body plan has been condensed antero-posteriorly and extended dorso-ventrally. What may have been the foot of the ancestor is modified into a complex set of appendages around the mouth. The sense organs are highly developed and include advanced eyes similar to those of vertebrates.