The guppy (Poecilia reticulata), also known as the millionfish or rainbow fish, is a member of the family Poeciliidae and, akin to most New World members of the family, is considered a livebearer. Male guppies, which are smaller than females, have ornamental caudal and dorsal fins. Wild guppies generally feed on a variety of food sources, including benthic algae and the larvae of aquatic insects.

Originating from northeast South America, it has been introduced to many environments and are now found all over the world; guppies are now one of the world's most widely distributed tropical fish and one of the most popular freshwater aquarium fish species. They are highly adaptable and thrive in many different environmental and ecological conditions. Guppies are used as a model organism in the fields of ecology, evolution, and behavioural studies.

Guppies were first described in Venezuela as Poecilia reticulata by Wilhelm Peters in 1859 and as Lebistes poecilioides in Barbados by De Filippi in 1861. It was named Girardinus guppii by Albert Günther in honour of Robert John Lechmere Guppy, who sent specimens of the species from Trinidad to the Natural History Museum in London. It was reclassified as Lebistes reticulatus by Regan in 1913. Then in 1963, Rosen and Bailey brought it back to its original name, Poecilia reticulata. While the taxonomy of the species was frequently changed and resulted in many synonyms, "guppy" remains the common name even as Girardinus guppii is now considered a junior synonym of Poecilia reticulata.

Guppies exhibit sexual dimorphism. While wild-type females are grey in body colour, males have splashes, spots, or stripes that can be any of a wide variety of colours. The development and exhibiting of colour patterns in male guppies is usually due to the amount of thyroid hormone that they contain. The thyroid hormones not only influence colour pattern, but control endocrine function in response to their environment. The size of guppies varies by gender, but males are typically 1.5–4 cm (0.6–1.6 in) long, while females are 3–7 cm (1.2–2.8 in) long.

A variety of fancy guppy strains are produced by breeders through selective breeding, characterised by different colours, patterns, shapes, and sizes of fins, such as snakeskin and grass varieties. Many domestic strains have morphological traits that are very distinct from the wild-type antecedents. Males and females of many domestic strains usually have larger body size and are much more lavishly ornamented than their wild-type antecedents.

Guppies are native to Antigua and Barbuda, Barbados, Brazil, Guyana, Trinidad and Tobago, and Venezuela. However, guppies have been introduced to many different countries on every continent except Antarctica. Sometimes this has occurred accidentally, but most often as a means of mosquito control. The guppies were expected to eat the mosquito larvae and help slow the spread of malaria, but in many cases, these guppies have had a negative impact on native fish year populations. Field studies reveal that guppies have colonised almost every freshwater body accessible to them in their natural ranges, especially in the streams located near the coastal fringes of mainland South America. Although not typically found there, guppies also have tolerance to brackish water and have colonised some brackish environments. They tend to be more abundant in smaller streams and pools than in large, deep, or fast-flowing rivers. They also are capable of being acclimated to full saltwater like their molly cousins.

Wild guppies feed on algal remains, diatoms, invertebrates, zooplankton, detritus, plant fragments, mineral particles, aquatic insect larvae, and other sources. Algal remains constitute the biggest proportion of wild guppy diet in most cases, but diets vary depending on the specific conditions of food availability in the habitat. For example, a study on wild Trinidad guppies showed that guppies collected from an oligotrophic upstream region (upper Aripo River) mainly consumed invertebrates, while guppies from a eutrophic downstream region (lower Tacarigua River) consumed mostly diatoms and mineral particles. Algae are less nutritious than invertebrates, and the guppies that feed mainly on algae have poor diets.

Guppies have also been observed eating native fishes' eggs, occasionally expressing cannibalism, also eating its own young, when kept in laboratory conditions.