Phoxinus phoxinus (known colloquially as the Eurasian minnow, minnow, Channel minnow, or common minnow) is a species of freshwater ray-finned fish belonging to the family Leuciscidae, which includes the daces, minnows and related fishes. It is the type species of genus Phoxinus. This taxon was formerly considered to be the only species in the genus Phoxinus found in Europe and to have a wide geohraphic range in Eurasia, however, it is now regarded as part of a species complex with new species being described across its former range and this species being restricted to Western Europe. It occurs predominantly in cool (12–20 °C (54–68 °F)) streams and well-oxygenated lakes and ponds. It is noted for being a gregarious species, shoaling in large numbers.

Phoxinus phoxinus was first formally described as Cyprinus phoxinus by Carl Linnaeus in the 10th edition of Systema Naturae published in 1758 with its type locality given as the River Agger north of Lohmar at 50°50'N, 7°12'E, in Nordrhein-Westfalen in Germany. In 1820 the French polymath and naturalist Constantine Samuel Rafinesque proposed the gneus Phoxinus, although he placed no species there but this species is the type species by absolute tautonymy. In 2007 Maurice Kottelat published a paper on the variation in populations of what were thought to be "common minnows" Ph. phoxinus sensu lato and revealed that what had been thought to be a single widespread Palearctic species was, in fact, a species complex. This species is classified within the genus Phoxinus, the Eurasian minnows, within the monotypic subfamily Phoxininae of the family Leuciscidae.

Phoxinus phoxinus is a small fish which reaches a maximum total length of 14 centimetres (5+1⁄2 in), but is normally around 7 centimetres (3 in) in length. It has 3 spines and 6–8 soft rays in its dorsal fin with 3 spines and 6–8 soft rays in its anal fin. Its spine is made up of 38–40 vertebrae. It is distinguished from similar species which occur in Europe by having the lateral line normally extending beyond the nase of the anal fin, by a line of vertically elongated blotches along the lateral line each with a depth equivalent to 1/3–1/2 of the body's depth at same position, these blotches often fuse to form a midlateral stripe, caudal peduncle has a depth of 2.6–3.1 times its length. The scales on the breast are patchy and the patches of scales are separated by unscaled areas although they are rarely connected by 1–2 rows of scales. Its snout length is 29–34% of the head length and is 1.1–1.4 times the diameter of the eye. The anal fin has its origin in front of the base of the last dorsal ray. The caudal fin consists of 19 soft rays. The back is normally brownish-green, and is separated from the whitish-gray underside by the lateral stripe or blotches described above,

Phoxinus phoxinus is indigenous to the river catchments draining into the North Sea and English Channel, from the Rhine system south to Normandy and in Great Britain. In Scotland it is regarded as an introduced non native species, and this is probably the case in Ireland. Introductions are likely to have occurred elsewhere, including Norway.

The common minnow is found in a wide variety of environments that have cold, well oxygenated water, often in syntopy with salmonids. These include small streams with fast currents, and, in the more northerly parts of its range, large lowland rivers. It also lives in still waters as varied as small mountain lakes to large, oligotrophic lakes. For spawning, it requires clean gravel areas in well oxygenated flowing water or where waves wash on lake shores. It also needs deep pools with low current to overwinter in, and these must have a coarse substrate among which the fish can hide.

Shoaling and schooling behavior of common minnows occur early in their development, as soon as they become capable of swimming. Shoaling behavior then increases and becomes dominant by three to four weeks after its emergence. This behavior generally benefits individual minnows by improving predator avoidance and foraging. However, there are also costs of living in groups such as increased competition for food and risk of infection. Shoaling behavior is modified depending on the situation such as presence of predators or resource availability.

The group formation of common minnows can be explained by the selfish herd effect proposed by W.D. Hamilton. According to the selfish herd theory, a group forms as individuals try to reduce their domain of danger by approaching others and continuously moving toward the center of the group where the risk of predation is the lowest. As the theory predicts, common minnows increase their shoaling behavior in response to increased predation pressure.

Common minnows can detect the predators' presence and communicate with their shoalmates by a chemical signal that is detected by olfactory nerves. The chemical, named Schreckstoff after a German word meaning "fear substance" by Karl von Frisch who first described it, is contained in specialized skin cells called alarm substance cells and is released from an injured or killed minnow. The shoalmates can detect the chemical and respond to the increased risk of predation.