The corn crake, corncrake or landrail (Crex crex) is a bird in the rail family. It breeds in Europe and Asia as far east as western China, and migrates to Africa for the Northern Hemisphere's winter. It is a medium-sized crake with buff- or grey-streaked brownish-black upperparts, chestnut markings on the wings, and blue-grey underparts with rust-coloured and white bars on the flanks and undertail. The strong bill is flesh-toned, the iris is pale brown, and the legs and feet are pale grey. Juveniles are similar in plumage to adults, and downy chicks are black, as with all rails. There are no subspecies, although individuals from the east of the breeding range tend to be slightly paler than their western counterparts. The male's call is a loud krek krek, from which the scientific name is derived. The corn crake is larger than its closest relative, the African crake, which shares its wintering range; that species is also darker-plumaged, and has a plainer face.

The corn crake's breeding habitat is grassland, particularly hayfields, and it uses similar environments on the wintering grounds. This secretive species builds a nest of grass leaves in a hollow in the ground and lays 6–14 cream-coloured eggs which are covered with rufous blotches. These hatch in 19–20 days, and the black precocial chicks fledge after about five weeks. This crake is in steep decline across much of its former breeding range because modern farming practices often destroy nests before breeding is completed. The corn crake is omnivorous but mainly feeds on invertebrates, the occasional small frog or mammal, and plant material including grass seed and cereal grain. Threats include dogs, cats, other introduced and feral mammals, large birds, various parasites and diseases.

Although numbers have declined steeply in western Europe, this bird is classed as least concern on the IUCN Red List because of its huge range and large, apparently stable, populations in Russia and Kazakhstan. Numbers in western China are more significant than previously thought, and conservation measures have facilitated an increased population in some countries which had suffered the greatest losses. Despite its elusive nature, the loud call has ensured the corn crake has been noted in literature, and garnered a range of local and dialect names.

The rails are a bird family comprising nearly 150 species. Although the origins of the group are lost in antiquity, the largest number of species and least specialised forms are found in the Old World, suggesting this family originated there. The taxonomy of the small crakes is complicated, but the closest relative of the corn crake is the African crake, which has been given its own genus, Crecopsis.

Corn crakes were first described by Carl Linnaeus in his 1758 10th edition of Systema Naturae as Rallus crex, but was subsequently moved to the genus Crex, created by German naturalist and ornithologist Johann Matthäus Bechstein in 1803, and named Crex pratensis. The earlier use of crex gives it priority over Bechstein's specific name pratensis, and leads to the current name of Crex crex. The binomial name, Crex crex, from the ancient Greek "κρεξ", is onomatopoeic, referring to the crake's repetitive grating call. The common name was formerly spelt as a single word, "corncrake", but the official version is now "corn crake". The English names refer to the species' habit of nesting in dry hay or cereal fields, rather than marshes used by most members of this family.

The corn crake is a medium-sized rail, 27–30 cm (11–12 in) long with a wingspan of 42–53 cm (17–21 in). Males weigh 165 g (5.8 oz) on average and females 145 g (5.1 oz). The adult male has the crown of its head and all of its upperparts brown-black in colour, streaked with buff or grey. The wing coverts are a distinctive chestnut colour with some white bars. The face, neck and breast are blue-grey, apart from a pale brown streak from the base of the bill to behind the eye, the belly is white, and the flanks, and undertail are barred with chestnut and white. The strong bill is flesh-coloured, the iris is pale brown, and the legs and feet are pale grey. Compared to the male, the female has warmer-toned upperparts and a narrower duller eye streak. Outside the breeding season, the upperparts of both sexes become darker and the underparts less grey. The juvenile is like the adult in appearance, but has a yellow tone to its upperparts, and the grey of the underparts is replaced with buff-brown. The chicks have black down, as with all rails. While there are no subspecies, all populations show great individual variation in colouring, and the birds gradually become paler and greyer towards the east of the range. Adults undergo a complete moult after breeding, which is normally finished by late August or early September, before migration to south eastern Africa. There is a pre-breeding partial moult prior to the return from Africa, mainly involving the plumage of the head, body and tail. Young birds have a head and body moult about five weeks after hatching.

The corn crake is sympatric with the African crake on the wintering grounds, but can be distinguished by its larger size, paler upperparts, tawny upperwing and different underparts pattern. In flight, it has longer, less rounded wings, and shallower wingbeats than its African relative, and shows a white leading edge to the inner wing. In both the breeding and wintering ranges it is unlikely to be confused with any other rails, since sympatric species are smaller, with white markings on the upperparts, different underparts patterns and shorter bills. A flying corn crake can resemble a gamebird, but its chestnut wing pattern and dangling legs are diagnostic.

On the breeding grounds, the male corn crake's advertising call is a loud, repetitive, grating krek krek normally delivered from a low perch with the bird's head and neck almost vertical and its bill wide open. The call can be heard from 1.5 km (0.93 mi) away, and serves to establish the breeding territory, attract females, and challenge intruding males. Slight differences in vocalisations mean that individual males can be distinguished by their calls. Early in the season, the call is given almost continuously at night, and often during the day, too. It may be repeated more than 20,000 times a night, with a peak between midnight and 3 am. The call has evolved to make a singing male's location clear, as this species hides in vegetation. The frequency of calling reduces after a few weeks but may intensify again near the end of the laying period before falling away towards the end of the breeding season. To attract males, mechanical imitations of their call can be produced by rubbing two pieces of wood or ribs, one of them with notches, or by flicking a credit card against a comb or zip-fastener. The male also has a growling call, given with the bill shut and used during aggressive interactions.