Clownfish or anemonefishes (genus Amphiprion) are saltwater fish found in the warm and tropical waters of the Indo-Pacific. They mainly inhabit coral reefs and have a distinctive colouration typically consisting of white vertical bars on a red, orange, yellow, brown or black background. Clownfish developed a symbiotic and mutually beneficial relationship with sea anemones, on which they rely for shelter and protection from predators. In turn, clownfish protect the anemone from anemone-eating fish, as well as clean and fan them, and attract beneficial microorganisms with their waste.

Clownfish are omnivorous and mostly feed on plankton. They live in groups consisting of a breeding female and male, along with some non-breeding individuals. Clownfish have a size-based dominance hierarchy with the breeding female ranking at the top, followed by the breeding male and then the largest non-breeder and so on. When the female disappears, the breeding male changes sex and takes her place while the others move up the hierarchy. During reproduction, the female deposits eggs on a rock near their anemone, and the male fertilises them. After hatching, clownfish disperse into the open ocean as larvae, eventually settling on the bottom and searching for an anemone host as juveniles.

The recognisable colour patterns and social nature of clownfish have contributed to their popularity. They are featured in the Disney/Pixar film Finding Nemo (2003) and are sought after in the aquarium trade. The ocellaris clownfish ranks among the most commonly traded marine fish. Many captive clownfish were taken from the wild and this has led to their decline. Clownfish are more numerous in marine protected areas, where collecting is forbidden. Other threats to populations include global warming, which causes ocean warming and acidification.

Clownfish are damselfishes (family Pomacentridae) in the genus Amphiprion (Greek amphi, 'on both sides' and prion 'saw'), which was coined by Marcus Elieser Bloch and Johann Gottlob Theaenus Schneider in 1801 using the red saddleback anemonefish as the type species. Georges Cuvier considered the maroon clownfish morphologically different enough to be placed in its own genus Premnas in 1816. The status of Premnas has been disputed over the years, switching between a synonym or subgenus of Amphiprion and being its own genus. In 2021, two expansive phylogenetic analyses of damselfishes found the species to be within Amphiprion, making Premnas a junior synonym. In 1975, ichthyologist Gerald R. Allen placed clownfish in their own subfamily Amphiprioninae. A 2009 genetic study suggested creating the tribe Amphiprionini for clownfish and moving them to the subfamily Pomacentrinae. Eschmeyer's Catalog of Fishes considers the genus to be part of Pomacentrinae though scientific papers still use Amphiprioninae.

The clownfish lineage diverged from that of other living damselfishes around 35 million years ago (mya) during the late Eocene, and the most recent common ancestor of extant species is dated around 10.5 mya during the Early Miocene. A 2014 study placed their origin in the waters of the Malay Archipelago. Clownfish experienced an increase in species diversification starting around 5 mya, with two major adaptive radiations; one centred on the Malay Archipelago and a later one in the waters of the western Indian Ocean. There is genetic evidence for high amounts of interbreeding between species through their evolutionary history. Clownfish speciation has been linked to their sea anemone hosts, species of which can be found in different habitats and thus drove ecological separation.

There are 28 living species of clownfish and two hybrids, the white-bonnet clownfish and Thielle's clownfish. More species possibly exist in the Pacific, likely as cryptic species. In 1972, Allen listed five major clades or complexes based on morphology; percula, akallopisos, ephippium, polymnus and clarkii with the maroon clownfish being in a clade on its own. A 2014 study lists two more major clades: Australian and Indian, with the maroon clownfish under the percula clade and the orange-fin anemonefish and wide-band anemonefish being single-species clades. A 2021 study placed the members of the clade polymnus in the Indian clade, while a 2025 study found the maroon clownfish to be a single-species clade.

The following cladogram of the 28 clownfish species is based on a 2025 genetic study, with clade/complex group labels based on a 2014 study.

Clownfish vary in size; the maroon clownfish can reach 160 mm (6.3 in) long while the orange clownfish reaches only 80 mm (3.1 in). Females are larger than males and the smallest individuals in a group are only 6–15 mm (0.24–0.59 in). Clownfish vary from oval-shaped to streamlined, and have rounded heads that lack scales between the snout and eyes. Teeth are present on both the oral and pharyngeal (throat) jaws but absent on the palate, and may be conical or chisel-shaped. Clownfish have saw-shaped edges along the operculum (gill covering) and under the eyes, which is the source of their genus name. The dorsal fin has 10 spines followed by 14–20 soft rays. It varies in shape; the ocellaris clownfish has a large recess between the spines and soft rays, while in the red saddleback anemonefish they are mostly continuous. Rays number 15–21 in the pectoral fins, five in the pelvic fins, 11–15 in the anal fin, and 14–15 in the caudal fin.