A piranha (/pɪˈrɑːnjəˌ -ræn/, or /pɪˈrɑːnə/; Portuguese: [piˈɾɐ̃ɲɐ]) is any of a number of freshwater fish species in the subfamily Serrasalminae, of the family Serrasalmidae, in the order Characiformes. These fish inhabit South American rivers, floodplains, lakes and reservoirs. Although often described as extremely predatory and mainly feeding on fish, their dietary habits vary extensively, and they will also take plant material, leading to their classification as omnivorous.

The name originates from Old Tupi pirãîa, from Proto-Tupi-Guarani *pirãj, being first attested in Vocabulário da Língua Brasílica, a 1585 Portuguese–Tupi bilingual dictionary, as a translation for scissors. The first description and attribution of this name to the fish would occur two years later, in the 1587 treatise Notícia do Brasil [pt] by Portuguese explorer Gabriel Soares de Sousa.

Piranha first appears in 1710 in English literature, borrowed from Portuguese.

Piranhas belong to the family Serrasalmidae, which includes closely related omnivorous fish such as pacus. Traditionally, only the four genera Pristobrycon, Pygocentrus, Pygopristis, and Serrasalmus are considered to be true piranhas, due to their specialized teeth. However, a recent analysis showed, if the piranha group is to be monophyletic, it should be restricted to Serrasalmus, Pygocentrus, and part of Pristobrycon, or expanded to include these taxa plus Pygopristis, Catoprion, and Pristobrycon striolatus. Pygopristis was found to be more closely related to Catoprion than the other three piranha genera.

Under more recent taxonomic treatments, essentially all members of the subfamily Serrasalminae within the family Serrasalmidae have "piranha" in their name, excluding the most basal members of the subfamily, Metynnis, which are referred to as "silver dollars".

The total number of piranha species is unknown and contested, and new species continue to be described. Estimates range from fewer than 30 to more than 60.

Piranhas are indigenous to the Amazon basin, in the Orinoco, in rivers of the Guianas, in the Paraguay–Paraná, and the São Francisco River systems, but there are major differences in the species richness. In a review where 38–39 piranha species were recognized, 25 were from the Amazon and 16 from Orinoco, while only three were present in Paraguay–Paraná and two in São Francisco. Most species are restricted to a single river system, but some (such as the red-bellied piranha) occur in several. Many species can occur together; for example, seven are found in Caño Maporal, a stream in Venezuela.

Aquarium piranhas have been unsuccessfully introduced into parts of the United States. In many cases, however, reported captures of piranhas are misidentifications of pacu (e.g., red-bellied pacu or Piaractus brachypomus is frequently misidentified as red-bellied piranha or Pygocentrus nattereri). Piranhas have also been discovered in the Kaptai Lake in southeast Bangladesh. Research is being carried out to establish how piranhas have moved to such distant corners of the world from their original habitat. Some rogue exotic fish traders are thought to have released them in the lake to avoid being caught by antipoaching forces. Piranhas were also spotted in the Lijiang River in China.