Ancistrus is a genus of nocturnal freshwater fish in the family Loricariidae of order Siluriformes, native to freshwater habitats in South America and Panama. Fish of this genus are common in the aquarium trade where they are known as bushynose or bristlenose catfish. In the aquarium hobby they are often referred to as bushynose or bristlenose plecos instead, but this may lead to confusion as "pleco" usually is used for Hypostomus plecostomus and its allies and is often used as a catchall term for any loricariids remotely resembling that species.

The type species is Ancistrus cirrhosus. This genus is the largest genus within the tribe Ancistrini.

The name ancistrus derives from the Ancient Greek agkistron "hook" – a reference to the form of the cheek odontodes. The genera Pristiancistrus, Thysanocara and Xenocara are now synonyms of Ancistrus.

Ancistrus species show all the typical features of the Loricariidae. This includes a body covered in bony plates and a ventral suckermouth. The feature most commonly associated with the genus are the fleshy tentacles found on the head in adult males; females may possess tentacles along the snout margin but they are smaller and they lack tentacles on the head. Tentacules, tentacles directly associated with odontodes, develop on the pectoral fin spine of the males of some species. Males also have evertible cheek odontodes which are less developed or absent in females. They also lack odontodes along the snout. In comparison with a typical loricariid (pleco), a bristlenose is typically shorter (4–6 inches or 15 cm or less), more flattened and fatter with a comparatively wider head. Colouration is typically mottled brown, grey or black. Small white or yellow spots are common. In contrast to many other loricariid 'plecos' (many of which regularly exceed a foot in captivity), the bristlenose plecos do not usually exceed six inches in length; they can thus be kept in relatively small tanks, contributing to their popularity in the aquarium hobby.

Ancistrus species are unusual among vertebrates in possessing an X0 sex-determination system, which is the prevailing method in many lineages of arthropod but is very rare elsewhere in the animal kingdom.

Ancistrus is one of the widest ranging Loricariid genera, and representatives are found in most areas where the family in general is present. Many species are found in the rivers and floodplain areas of the Amazon basin, but there are also species elsewhere in tropical South America, as well as species on the island of Trinidad and two species, A. centrolepis and A. chagresi in Panama. Three species are true cavefish (troglobites): A. cryptophthalmus, A. galani and A. formoso. These are the only known loricariids that possess adaptions for a subterranean lifestyle, such as reduced pigmentation (appearing overall whitish) and eyes.

In the wild, Ancistrus species prefer flowing water, so tanks with flow are typically used when keeping them in captivity. Since they are bottom-dwelling fish, substrate is considered preferable to a bare tank floor. Mixtures of gravel, dirt, and clay are frequently used as substrate for Ancistrus.

The aquarium size that is considered by some to be a minimum for fish of this genus is 25 gallons.^{[self-published source?]}