The Siamese cat (Thai: แมวไทย, Maeo Thai; แมวสยาม, Maeo Sayam) is one of the first distinctly recognised breeds of Asian cat. Derived from the Wichianmat landrace, one of several varieties of cats native to Thailand (formerly known as Siam), the original Siamese became one of the most popular breeds in Europe and North America in the 19th century.^[1] Siamese cats have a distinctive colourpoint coat, resulting from a temperature-sensitive type of albinism.

The carefully refined, more extreme-featured, modern-style Siamese is characterised by blue almond-shaped eyes; a triangular head shape; large ears; an elongated, slender, and muscular body; and various forms of point colouration. Other than point colouration, the modern-style Siamese bears little resemblance to the original foundation stock, and the more moderate, traditional, or "old-style" Siamese, with a much rounder head and body, has been re-established by multiple registries as the Thai cat. Both Siamese and Thai cats are separately selectively bred by breeders, and pedigreed in multiple major cat fancier and breeder organisations. Similar to other officially recognised cat breeds, the term "Siamese" or "Thai" is only meant to be used for cats from this specific breed, which are by definition all purebred cats with a known and formally registered ancestry, also known as the cat's pedigree or "paperwork".^[2][3]

The Siamese (sometimes in the traditional form) is among the foundation stock of several other breeds developed by crossbreeding with other cats. Some examples are the Oriental Shorthair and Colourpoint Shorthair, developed to expand the range of coat patterns; the long-haired variant most often dubbed the Himalayan; and hair-mutation breeds, including the Cornish Rex, Sphynx, Peterbald, and blue-point Siamese cat. The long-haired Siamese is recognised internationally as a Balinese cat.