Typically, true piranhas are limited to three species of genus Pygocentrus: P. piraya, P. nattereri, and P. cariba. Historically, however, other classifications have expanded the group to include four genera: Pristobrycon, Pygocentrus, Pygopristis, and Serrasalmus, based largely on the single row of sharp triangular teeth all members display.

There are only four families who are considered to be true piranhas. This includes pristobrycon, pygocentrus, pygopristis, and serrasalmus. These four species have specialized teeth, and the total number of piranha fish species is unknown, and scientists continue to contest on what should be considered piranha and what should not.

Traditionally, only the four genera Pristobrycon, Pygocentrus, Pygopristis, and Serrasalmus are considered to be true piranhas, due to their specialized teeth.

Piranha is the common name for various species of fish within the family Serrasalmidae or the subfamily Serrasalminae that are also in the tetra family Characidae of the Characiformes order. Piranha species are found in several genera, including: Catoprion Pristobrycon Pygocentrus Pygopristis Serrasalmus