The Siamese fighting fish (Betta splendens) is one of the most popular species of freshwater aquarium fish. It is a member of the gourami family (family Osphronemidae) of order Perciformes, but was formerly classified among the Anabantidae. It is native to the Mekong basin in Southeast Asia. The Siamese fighting fish has been the subject of movies, arts, poetry, and sclupture.

The natural colouration of the Siamese fighting fish or Betta splendens is a dull green and brown, and the fins of wild specimens are relatively short; brilliantly-coloured and longer-finned varieties have, however, been developed by breeders.

In the wild, the Siamese fighting fish inhabits standing or slow-moving water, including floodplains and rice paddys, at temperatures of 24–30°C (75–86°F). Carnivorous, it feeds on zooplankton and mosquito and other insect larvae. They are indeed the majestic fish that is well known in bowls and aquariums throughout the world

Betta splendens grow to an overall length of approximately 6 cm (2.5 in), and has a life span of up to two years.

The Siamese fighting fish is often called by its generic name, which is then treated as a common name beta, and are therefore know as beta fish.