Pacu (Portuguese pronunciation: [paˈku]) is a common name used to refer to several species of omnivorous South American freshwater serrasalmid fish related to piranhas. Pacu and piranha do not have similar teeth, the main difference being jaw alignment; piranha have pointed, razor-sharp teeth in a pronounced underbite, whereas pacu have squarer, straighter teeth and a less severe underbite or a slight overbite. Pacu, unlike piranha, mainly feed on plant material and not flesh or scales. Additionally, the pacu can reach much larger sizes than piranha, up to 1.08 m (3 ft 6+1⁄2 in) in total length and 40 kg (88 lb) in weight.

The common name pacu is generally applied to fish classified under the below listed genera. Among these, several genera contain species where commonly used English names include the word pacu, as listed.

Each of these groups contains one or more separate species. For example, the fish often found in pet stores, known as the black pacu and red-bellied pacu, typically belong to the species Colossoma macropomum and Piaractus brachypomus, respectively. A species popular among fish farmers is Piaractus mesopotamicus, also known as Paraná River pacu or small-scaled pacu.

Pacu is a term of Tupi-Guaraní origin. When the large fish of the genus Colossoma entered the aquarium trade in the U.S. and other countries, they were labeled pacu. In the Brazilian Amazon, the term pacu is generally reserved to smaller and medium-sized fish in the Metynnis, Mylossoma, Mylesinus and Myleus genera. Colossoma macropomum are known as tambaqui, whereas Piaractus brachypomus is known as pirapitinga. In Peru, both of the species (Colossoma macropomum and Piaractus brachypomum) are called pacú and gamitana. Piaractus mesopotamicus of the Paraná-Paraguay basin is also called pacú in Paraguay and Argentina.

Pacu, along with their piranha relatives, are a characin fish, meaning a kind of tetra, belonging to the Characiformes order. The ongoing classification of these fish is difficult and often contentious, with ichthyologists basing ranks according to characteristics that may overlap irregularly. DNA research sometimes confounds rather than clarifies species ranking. Ultimately, classifications can be rather arbitrary.

Pacu, along with piranha, are currently further classified into the family Serrasalmidae (formerly a subfamily of Characidae). Serrasalmidae means "serrated salmon family" and refers to the serrated keel running along the belly of these fish. However, dental characteristics and feeding habits further separate the two groups from each other.

Pacus are native to tropical and subtropical South America. They inhabit rivers, lakes, floodplains and flooded forests in the Amazon, Orinoco, São Francisco River and Río de la Plata Basins, as well as rivers in the Guianas. Here they form part of the highly diverse Neotropical fish fauna. Their habitat preferences varies significantly depending on the exact species. Several species are migratory.

Pacus have been introduced to the wild in many places outside their native range, both in South America and other continents. They are sometimes released to increase the local fishing, but others are released by aquarists when they outgrow their aquarium. This is illegal in many countries and strongly advised against. When becoming established they can be invasive species that damage the local ecosystem. In many regions where individuals have been seen they are unlikely to survive long-term because of temperature. Among the large pacu species (Colossoma and Piaractus), the most cold-adapted species is Piaractus mesopotamicus, which tolerates water down to 15 °C (59 °F), but stops feeding below 18 °C (64 °F).