The guppy , also known as millionfish and rainbow fish, is one of the world's most widely distributed tropical fish, and one of the most popular freshwater aquarium fish species. It is a member of the Poeciliidae family and, like almost all American members of the family, is live-bearing. Guppies, whose natural range is in northeast South America, were introduced to many habitats and are now found all over the world. They are highly adaptable and thrive in many different environmental and ecological conditions. Male guppies, which are smaller than females, have ornamental caudal and dorsal fins, while females are duller in colour. Wild guppies generally feed on a variety of food sources, including benthic algae and aquatic insect larvae. Guppies are used as a model organism in the field of ecology, evolution, and behavioural studies.

The Greater Underwater Propulsion Power Program (GUPPY) was initiated by the United States Navy after World War II to improve the submerged speed, maneuverability, and endurance of its submarines. (The "Y" in the acronym was added for pronounceability.)