Serrasalmus is a genus of freshwater ray-finned fishes belonging to the family Serrasalmidae, which includes the pacus, piranhas and related fishes. They are collectively known as pirambebas; the "typical" piranhas like the piraya piranha are nowadays placed in Pygocentrus. Like all piranhas, Serrasalmus are native to South America.

One species, S. humeralis, was able to temporarily establish a breeding population in Florida before being eradicated in 1981.

Serrasalmus species have sharp teeth and generally have a compressed rhomboid shape.^{[verification needed]} In some, the shape is more ovoid, particularly in old specimens. There is also a high variation in color patterns found within this genus.^{[verification needed]} Some Serrasalmus species can exceed 20 in or 510 mm (S. manueli and S. rhombeus, according to OPEFE), placing them among the largest Serrasalmidae.^{[citation needed]}

Serrasalmus species are primarily piscivorous, with varying degrees of propensity for omnivory depending on the species. Plant material ingested may include fruits and seeds.

Pericentric inversions are likely responsible for many of the chromosomal differences in Serrasalmus.

Middle Miocene-aged fossil tooth remains of a serrasalmid potentially referable to Serrasalmus are known from the Pebas Formation of Peru. The genus otherwise does not have a fossil record.

These are the recognized species in this genus: