Megapiranha is an extinct serrasalmid characin fish from the Late Miocene (8–10 million years ago) Ituzaingó Formation of Argentina, described in 2009.

[3] The holotype of Megapiranha was discovered in an unknown locality of the Ituzaingó Formation, Argentina, in the early 20th century near the towns of Paraná and Villa Urquiza.

The specimen, a fragment of the animal's premaxilla containing several teeth, was later rediscovered by Alberto Luis Cione in the collection of the Museo de La Plata.

[4] The name Megapiranha is a combination of the word "mega" in reference to the animal's large size and piranha, a common name for typically carnivorous members of Serrasalmidae.

[4] Based on the size of the holotype, Megapiranha has originally been estimated to have reached a length of 95–128 cm (37–50 in) and a weight of 73 kg (161 lb), larger than any other member of the family, living or extinct.

[4] Later research using Serrasalmus rhombeus as a basis arrived at a more conservative size estimate of 71 cm (28 in) long and 10 kg (22 lb) heavy.

Part of their reasoning for this is the broad range of diets found within serrasalmids, including many herbivorous and omnivorous forms in addition to carnivores, as well as the highly specialised wimple piranha which feeds primarily on the scales of other fish.

[4] In 2012 Justin R. Grubich and colleagues suggest that the dentition of Megapiranha may have been a transitional form between feeding on hard prey and specialising in slicing flesh.

[3] Tests using a bronze-alloy replica of Megapiranha's dentition showed that it would be able to penetrate the thick outer layer of a bovine femur, the shell of a turtle and the armor of certain catfish species.

They conclude by suggesting that Megapiranha could have hypothetically fed on hard-shelled animals such as turtles, armored catfish and even attacked larger mammals.