Lampreys /ˈlæmpreɪz/ (sometimes inaccurately called lamprey eels) are a group of jawless fish composing the order Petromyzontiformes /ˌpɛtroʊmɪˈzɒntɪfɔːrmiːz/, sole order in the class Petromyzontida. The adult lamprey is characterized by a toothed, funnel-like sucking mouth. The common name "lamprey" is probably derived from Latin lampetra, which may mean "stone licker" (lambere "to lick" + petra "stone"), though the etymology is uncertain. "Lamprey" is sometimes seen for the plural form.

About 38 extant species of lampreys are known, with around seven known extinct species. They are classified in three families—two small families in the Southern Hemisphere (Geotriidae, Mordaciidae) and one large family in the Northern Hemisphere (Petromyzontidae).

Genetic evidence suggests that lampreys are more closely related to hagfish, the only other living group of jawless fish, than they are to jawed vertebrates, forming the superclass Cyclostomi. The oldest fossils of stem-group lampreys are from the latest Devonian, around 360 million years ago, with modern-looking forms only appearing during the Jurassic, around 163 million years ago, with the modern families likely splitting from each other sometime between the Middle Jurassic and the end of the Cretaceous.

Modern lampreys spend the majority of their lives in the juvenile "ammocoete" stage, where they burrow into the sediment and filter feed. Adult carnivorous lampreys are the most well-known species, and feed by boring into the flesh of other fish (or in rare cases marine mammals) to consume flesh and/or blood; but only 18 species of lampreys engage in this predatory lifestyle (with Caspiomyzon suggested to feed on carrion rather than live prey). Of the 18 carnivorous species, nine migrate from saltwater to freshwater to breed (some of them also have freshwater populations), and nine live exclusively in freshwater. All noncarnivorous forms are freshwater species. Adults of the noncarnivorous species do not feed; they live on reserves acquired as ammocoetes.

Lampreys live mostly in coastal and fresh waters and are found in most temperate regions. Some species (e.g. Geotria australis, Petromyzon marinus, and Entosphenus tridentatus) travel significant distances in the open ocean, as evidenced by their lack of reproductive isolation between populations. Other species are found in land-locked lakes. Their larvae (ammocoetes) have a low tolerance for high water temperatures, which may explain why they are not distributed in the tropics.

Lamprey distribution may be adversely affected by river habitat loss, overfishing, and pollution. In Britain, at the time of the 11th-century Norman Conquest of England, lampreys were found as far upstream in the River Thames as Petersham. The reduction of pollution in the Thames and River Wear has led to recent sightings in London and Chester-le-Street.

Distribution of lampreys may also be adversely affected by dams and other construction projects due to disruption of migration routes and obstruction of access to spawning grounds. Conversely, the construction of artificial channels has exposed new habitats for colonisation, notably in North America, where sea lampreys have become a significant introduced pest in the Great Lakes. Active programs to control lampreys are undergoing modifications due to concerns of drinking-water quality in some areas.

Adults superficially resemble eels in that they have scaleless, elongated bodies, with the largest species, the sea lamprey having a maximum body length around 1.2 m (3.9 ft). Lacking paired fins, adult lampreys have one nostril atop the head and seven gill pores on each side of the head.