The Ruddy Turnstone (Arenaria interpres) is a small wading bird, one of two species of turnstone in the genus Arenaria. It is now classified in the sandpiper family Scolopacidae but was formerly sometimes placed in the plover family Charadriidae. It is a highly migratory bird, breeding in northern parts of Eurasia and North America and flying south to winter on coastlines almost worldwide. It is the only species of turnstone in much of its range and is often known simply as Turnstone.

It is a fairly small and stocky bird, 22-24 centimetres long with a wingspan of 50-57 centimetres and a weight of 85-150 grams. The dark, wedge-shaped bill is 2-2.5 centimetres long and slightly upturned. The legs are fairly short at 3.5 centimetres and are bright orange.

At all seasons, the plumage is dominated by a harlequin-like pattern of black and white. Breeding birds have reddish-brown upperparts with black markings. The head is mainly white with black streaks on the crown and a black pattern on the face. The breast is mainly black apart from a white patch on the sides. The rest of the underparts are white. In flight it reveals a white wingbar, white patch near the base of the wing and white lower back, rump and tail with dark bands on the uppertail-coverts and near the tip of the tail. The female is slightly duller than the male and has a browner head with more streaking.

Non-breeding adults are duller than breeding birds and have dark grey-brown upperparts with black mottling and a dark head with little white. Juvenile birds have a pale brown head and pale fringes to the upperpart feathers creating a scaly impression.

Birds of the subspecies morinella are smaller with darker upperparts and less streaking on the crown.

The Ruddy Turnstone has a staccato, rattling call and also a chattering alarm-call which is mainly given during the breeding season.

It breeds in northern latitudes, usually no more than a few kilometres from the sea. The subspecies A. i. morinella occurs in northern Alaska and in arctic Canada as far east as Baffin Island. A. i. interpres breeds in western Alaska, Ellesmere Island, Greenland, Norway, Denmark, Sweden , Finland, Estonia and northern Russia. It formerly bred on the Baltic coast of Germany and has possibly bred in Scotland and the Faroe Islands.

In the Americas the species winters on coastlines from Washington State and Massachusetts southwards to the southern tip of South America although it is scarce in southern parts of Chile and Argentina and is only an unconfirmed vagrant in the Falkland Islands. In Europe it winters in western regions from Iceland, Norway and Denmark southwards. Only small numbers are found on Mediterranean coasts. In Africa it is common all the way down to South Africa with good numbers on many offshore islands. In Asia it is widespread in the south with birds wintering as far north as southern China and Japan (mainly in the Ryukyu Islands). It occurs south to Tasmania and New Zealand and is present on many Pacific islands. In many parts of the wintering range, some non-breeding birds remain all year round.