The red knot or just knot (Calidris canutus) is a medium-sized shorebird which breeds in tundra and the Arctic Cordillera in the far north of Canada, Europe, and Russia. It is a large member of the Calidris sandpipers, second only to the great knot. Six subspecies are recognised.

Their diet varies according to season; arthropods and larvae are the preferred food items at the breeding grounds, while various hard-shelled molluscs are consumed at other feeding sites at other times. North American breeders migrate to coastal areas in Europe and South America, while the Eurasian populations winter in Africa, Papua New Guinea, Australia, and New Zealand. This species forms enormous flocks when not breeding.

The red knot was first described by Carl Linnaeus in his landmark 1758 10th edition of Systema Naturae as Tringa canutus. One theory is that it gets its name and species epithet from King Cnut; the name would refer to the knot's foraging along the tide line and the story of Cnut and the tide. There appears to be no historical foundation for this etymology. Another etymology is that the name is onomatopoeic, based on the bird's grunting call note.

The red knot and the great knot were originally the only two species placed in the genus Calidris, but many other species of sandpiper were subsequently added. A 2004 study found that the genus was polyphyletic and that the closest relative of the two knot species is the surfbird (Calidris virgata).

There are six subspecies, in order of size;

Studies based on mitochondrial sequence divergence and models of paleoclimatic changes during the glacial cycles suggest that C. c. canutus is the most basal population, separating about 20,000 years ago (95% confidence interval: 60,000–4,000 years ago) with two distinct lineages of the American and Siberian breeders emerging about 12,000 years ago (with a 95% confidence interval: 45,000–3,500 years ago).

In the breeding season, the red knot has a circumpolar distribution in the high Arctic, then migrates to coasts around the world to winter at latitudes between 60° N (Shetland) to 55° S (Tierra del Fuego). The red knot has one of the longest migrations of any landbird. Every year some birds of C. c. rufa travel more than 14,000 km (9,000 mi) from the Arctic to the southern tip of South America and repeat the trip in reverse. The exact migration routes and wintering grounds of individual subspecies are still somewhat uncertain. The nominate race C. c. canutus breeds in the Taymyr Peninsula and possibly Yakutia, and migrates to Western Europe and then down to winter in western and southern Africa. C. c. rogersi breeds in the Chukchi Peninsula in eastern Siberia, and winters in eastern Australia and New Zealand. Small and declining numbers of C. c. rogersi (but possibly of the later described C. c. piersmai) winter in the mudflats in the Gulf of Mannar and on the eastern coast of India. The recently split race C. c. piersmai breeds in the New Siberian Islands and winters in north-western Australia. C. c. roselaari breeds in Wrangel Island in Siberia and north-western Alaska, and it apparently winters in Florida, Panama and Venezuela. C. c. rufa breeds in the Canadian low Arctic, and winters South America, and C. c. islandica breeds in the Canadian high Arctic and Greenland, and migrates through Iceland to winter in Western Europe.

Birds wintering in west Africa were found to restrict their daily foraging to a range of just 2–16 km² (0.77–6.18 sq mi) of intertidal area and roosted a single site for several months. In temperate regions such as the Wadden Sea they have been found to change roost sites each week and their feeding range may be as much as 800 km² (310 sq mi) during the course of a week.