A viperfish is any species of marine fish in the genus Chauliodus. Viperfish are mostly found in the mesopelagic zone and are characterized by long, needle-like teeth and hinged lower jaws. A typical viperfish grows to lengths of 30 cm (12 in). Viperfishes undergo diel vertical migration and are found all around the world in tropical and temperate oceans. Viperfishes are capable of bioluminescence and possess photophores along the ventral side of their body, likely used to camouflage them by blending in with the less than 1% of sunlight that reaches to below 200 meters depth.

Viperfish live in meso- and bathypelagic environments and have been found dominating submarine calderas such as the Kurose Hole, which is the site with the highest Chauliodus density known in the world. Viperfish also engage in diel vertical migration, meaning they migrate up into more productive waters during the night to feed. However, it is likely that only part of the total population of viperfishes engages in diel vertical migration on any given night, which could be due to their slow metabolism, i.e. they likely do not have to feed every night. Temperature is another restricting factor in viperfish's vertical distribution in the ocean. Depth is restricted by temperature, and the upper thermal limit of viperfish is 12° to 15 °C. In tropical waters, viperfish tend to stay in the deep layers and not migrate much, while in temperate waters viperfish are more actively migrating and even interacting with epipelagic predators.

Chauliodus species are recognized by their large, fang-like teeth. They are so long that they would pierce the brain of the fish if misaligned.

One species of viperfish, C. sloani, have a sampled standard length of 64.0 to 260.0 mm, with a mean SL of 120.3mm. The same species has a mean weight of 5.66 grams. Representatives from Chauliodus pammelas and Chauliodus sloani display a size-based depth differential. Individuals of a lesser mass are found at shallower depths and individuals of larger mass are found at deeper depths, below 500 meters. However, at nighttime larger viperfish can be found in shallower depths.

The eyes of Chauliodus sloani maintain a constant size and proportion throughout growth of the fish. In the retina, several rows of rod cell "banks" grow upon each other, increasing in number with size of the fish. This opposed the typical vertebrate retina, which only has one layer of receptors.

The first dorsal ray of Chauliodus is elongated, hinged, and connected via musculature; allowing it to swing forward. The tip of this ray has light organs.

Viperfish are covered in scales, arranged in five longitudinal rows. These scales are easily shed, and may be dissolved in preservation fluid when specimens are preserved, leading some publications to describe them as scaleless. Beneath the scales, the skin is covered with hexagonal pigment patterns and an opalescent, slimy substance.

Extremely large, fang-like teeth give the fish a slightly protruded lower jaw.