Characiformes /ˈkærəsɪfɔːrmiːz/ is an order of ray-finned fish, comprising the characins and their allies. Grouped in 18 recognized families, more than 2000 different species are described, including the well-known piranha and tetras. Characins are most diverse in the Neotropics, where they are found in lakes and rivers throughout most of South and Central America. At least 209 species of characins are found in Africa, including the distichodontids, citharinids, alestids, and hepsetids. The rest of the characins originate from the Americas.

A few characins become quite large, and are important as food or game. Most, however, are small shoaling fish (or forage fish). Many species commonly called tetras are popular in aquaria because of their bright colors, general hardiness, and tolerance towards other fish in community tanks.

Characins possess a Weberian apparatus, a series of bony parts connecting the swim bladder and inner ear. Superficially, the Characiformes somewhat resemble their relatives of the order Cypriniformes, but may have a small, fleshy adipose fin between the dorsal fin and tail. Most species have teeth within the mouth, since they are often carnivorous. The body is almost always covered in well-defined scales. The mouth is also usually not truly protractile.

The largest characins are Hydrocynus goliath (the goliath tigerfish of Alestidae), Salminus brasiliensis or Salminus franciscanus (golden dourado of Bryconidae), and Hoplias aimara (a traíra or wolffish of Erythrinidae), all of which are over 1 m (3.3 ft) long. Many members are under 3 cm (1.2 in), and the smallest in size is about 1.4–1.7 cm (0.55–0.67 in) in the Bolivian pygmy blue characin, Xenurobrycon polyancistrus.

The Characiformes form part of a series called the Otophysi within the superorder Ostariophysi. The Otophysi contain three other orders, Cypriniformes, Siluriformes, and Gymnotiformes. The Characiformes form a group known as the Characiphysi with the Siluriformes and Gymnotiformes. The order Characiformes is the sister group to the orders Siluriformes and Gymnotiformes, though this has been debated in light of recent molecular evidence.

Originally, the characiformes were all grouped within a single family, the Characidae. Since then, 18 different families have been separated out. However, classification varies somewhat, and the most recent (2011) study confirms the circumscribed Characidae as monophyletic. Currently, 18 families, about 270 genera, and at least 1674 species are known.

The suborder Citharinoidei, which contains the families Distichodontidae and Citharinidae, is considered the sister group to the rest of the characins, suborder Characoidei. This group has a very ancient divergence from the rest of the Characiformes, dating back to the Early Cretaceous or earlier, and it has been suggested that it be better treated as its own order, the Cithariniformes.

Betancur et al (2018) & Melo et al (2022) recognise some infraordinal and superfamilial divisions but these are not recognised by Eschmeyer's Catalog of Fishes, which the following classification is based upon (2025 edition):