The Sandwich Tern (Sterna sandvicensis) is a seabird of the tern family Sternidae. It is sometimes placed in an alternative genus as Thalasseus sandvicensis.

This bird has three races:

• S. s. sandvicensis breeds on the Atlantic and Mediterranean coasts of Europe, and winters off western Africa and Arabia.
• The marginally smaller S. s. acuflavida breeds on Atlantic coasts of North America, wintering in the Caribbean and further south, and has wandered to western Europe.
• Yellow-billed S. s. eurygnatha (sometimes treated as a separate species, Cayenne Tern S. eurygnatha) breeds on the Atlantic coast of South America from Argentina north to the Caribbean, intergrading with S. s. acuflavida in the north of its range.
  • Sandwich Tern is also very closely related to Lesser Crested Tern S. bengalensis, Chinese Crested Tern S. bernsteini, and Elegant Tern S. elegans, and has been known to interbreed with Lesser Crested.

This species breeds in very dense colonies on coasts and islands, and exceptionally inland on suitable large freshwater lakes close to the coast. It nests in a ground scrape and lays one to three eggs. Unlike some of the smaller white terns, it is not very aggressive toward potential predators, relying on the sheer density of the nests (often only 20-30cm apart) and nesting close to other more aggressive species such as Arctic Terns and Black-headed Gulls to avoid predation.

Like all Sterna terns, Sandwich Tern feeds by plunge-diving for fish, almost invariably from the sea. It usually dives directly, and not from the "stepped-hover" favoured by Arctic Tern. The offering of fish by the male to the female is part of the courtship display.

This is a medium-large tern, 37-43cm long with a 85-97cm wingspan, which is unlikely to be confused within most of its range, although the South American race could be confused with the Elegant Tern.

The Sandwich Tern's thin sharp bill is black with a yellow tip, except in the yellow or orange billed South American race. Its short legs are black. Its upperwings are pale grey and its underparts white, and this tern looks very pale in flight, although the primary flight feathers darken during the summer. The call is a characteristic loud grating noise.

The Lesser Crested Tern and Elegant Tern differ in having all-orange bills; Lesser Crested also differs in having a grey rump and marginally stouter bill, and Elegant in having a slightly longer, slenderer bill. Chinese Crested Tern is the most similar to Sandwich, but has a reversal of the bill colour, yellow with a black tip; it does not overlap in range with Sandwich Tern so confusion is unlikely.

In winter, the Sandwich Tern's forehead becomes white. Juvenile Sandwich Terns have a scaly appearance like juvenile Roseate Terns.