The arapaima, pirarucu, or paiche is a large species of bonytongue in the genus Arapaima native to the Amazon basin of South America. Arapaima is the type genus of the subfamily Arapaiminae within the family Osteoglossidae, which means they are the same family as Silver Arowanas.

The species is among the largest known freshwater fish, commonly measuring from 7 to 10 feet, with rare reports of up to 15 feet being recorded. The Arapaima 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. 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.

This is one of the few freshwater fish truly deserving the title "River Monster"