Arapaima gigas, also known simply as Arapaima, pirarucu, or paiche, is a species of arapaima native to the basin of the Amazon River. Once believed to be the sole species in the genus, it is among the largest freshwater fish. It is the best-known species of Arapaimas.

Arapaima gigas was originally regarded as the only species in the genus Arapaima (monotypic), but the subsequent identification of further species, together with the rarity of specimens and the loss of several type specimens, has led to some uncertainty regarding classification within the genus and the identity of described individuals. Recently, some studies have added extra species to the genus due to common morphological features, including Arapaima leptosoma and Arapaima agassizii. These speciation events may have occurred due to evidence of "long-distance marine or geodispersal" from similar morphological traits in both fossilized and living specimen within the genus. Overall, it is unclear if Arapaima gigas truly is the only species in its genus.

The species is among the largest known freshwater fish, commonly measuring 200 cm (79 in) and reportedly exceptionally reaching lengths of up to 450 cm (15 ft). Adults may weigh up to 200 kg (440 lb). Of the total body weight, head weight is typically 10.3% to 12.8%.

It is an ancient fish, belonging to a group of primitive carnivorous bony-tongued fishes. A. gigas has a streamlined body with dorsal and anal fins set well back towards the tail. While the body is mainly gray to gray-green, its Brazilian local name pirarucu derives from an indigenous word for "red fish", thought to refer to either the red flecks on the scales towards the tail, or the reddish-orange color of its meat. Red coloration typically shows up in males after sexual maturation, with up to 70% of the total body plan in males having red coloration.

The fish have "flexible, armor-like scales" made up of "a hard, mineralized outer layer" and "a tough-but-flexible inner layer" that help protect it from attacks by piranhas. The scales, which are typically 5–7cm but can reach 10cm in larger individuals, are built from collagen fibers in a layered structure. On the body, they overlap each other to form the armor-like characteristic.

A. gigas is native to freshwater in the basin of the Amazon River and Tocantins–Araguaia (Brazil) river basins; it is known to occur in Brazil and Peru. In Bolivia and Peru it is known as paiche, and is considered an invasive species in most parts, affecting local native species and the ecosystem. Other small populations have been found in the Iquitos region of Peru, and four distinct rivers in Ecuador, Colombia and Guyana.

Its populations are increasing and becoming more common in its non-native range, yet is rapidly declining in its native range. It was first found in 1976, introduced into the Madre de Dios region in Peru, for aquaculture. In about a decade, arapaimas were found in Bolivian waters due to the connection of the Madre de Dios basin to the Beni River in Bolivia. Rising water of aquaculture ponds allowed fish to escape into the watershed and establishing populations outside of containment.

Current distribution in Bolivia is characterized by 70 registered distribution points, including small rivers, lagoons, and streams between the Beni and Madre de Dios (Peru) rivers. Historical reports of arapaima sightings started north near the border of Peru, then traveled downstream as populations began to establish and spatially separate. In less than 30 years, the species has invaded the Madre de Dios and Beni watersheds.