The Eurasian sparrowhawk (Accipiter nisus), also known as the northern sparrowhawk or simply the sparrowhawk, is a small bird of prey in the family Accipitridae. Adult male Eurasian sparrowhawks have bluish grey upperparts and orange-barred underparts; females and juveniles are brown above with brown barring below. The female is up to 25% larger than the male – one of the greatest size differences between the sexes in any bird species. Though it is a predator which specialises in catching woodland birds, the Eurasian sparrowhawk can be found in any habitat and often hunts garden birds in towns and cities. Males tend to take smaller birds, including tits, finches and sparrows; females catch primarily thrushes and starlings but are capable of killing birds weighing 500 g (18 oz) or more.

The Eurasian sparrowhawk is found throughout the temperate and subtropical parts of the Old World; whilst birds from the northern parts of the range migrate south for winter, their southern counterparts remain resident or make dispersive movements. Eurasian sparrowhawks breed in suitable woodland of any type, with the nest, measuring up to 60 cm (24 in) across, built using twigs in a tree. Four or five pale blue, brown-spotted eggs are laid; the success of the breeding attempt is dependent on the female maintaining a high weight while the male brings her food. The chicks hatch after 33 days and fledge after 24 to 28 days.

The probability of a juvenile surviving its first year is 34%, with 69% of adults surviving from one year to the next. Mortality in young males is greater than that of young females and the typical lifespan is four years. This species is now one of the most common birds of prey in Europe, although the population crashed after the Second World War. Organochlorine insecticides used to treat seeds before sowing built up in the bird population, and the concentrations in Eurasian sparrowhawks were enough to kill some outright and incapacitate others; affected birds laid eggs with fragile shells, which broke during incubation. However, its population recovered after the chemicals were banned, and it is now relatively common, classified as being of least concern by BirdLife International.

The Eurasian sparrowhawk's hunting behaviour has brought it into conflict with humans for hundreds of years, particularly racing pigeon owners and people rearing poultry and gamebirds. It has also been blamed for decreases in passerine populations. Studies of racing-pigeon deaths found that Eurasian sparrowhawks were responsible for less than 1%. Falconers have utilised the Eurasian sparrowhawk since at least the 16th century; although the species has a reputation for being difficult to train, it is also praised for its courage. The species features in Teutonic mythology and is mentioned in works by writers including William Shakespeare, Alfred, Lord Tennyson and Ted Hughes.

Within the family Accipitridae, the Eurasian sparrowhawk is a member of the large genus Accipiter, which consists of small to medium-sized woodland hawks. Most of the Old World members of the genus are called sparrowhawks or goshawks. The species' name dates back to the Middle English word sperhauk and Old English spearhafoc, a hawk which hunts sparrows. The Old Norse name for the Eurasian sparrowhawk, sparrhaukr, was thought to have been coined by Vikings who encountered falconry in England. English folk names for the Eurasian sparrowhawk include blue hawk, referring to the adult male's colouration, as well as hedge hawk, spar hawk, spur hawk and stone falcon.

The Eurasian sparrowhawk was described by Carl Linnaeus in his 1758 landmark 10th edition of Systema Naturae as Falco nisus, but moved to its present genus by French zoologist Mathurin Jacques Brisson in 1760. The current scientific name is derived from the Latin accipiter, meaning 'hawk', and nisus, the sparrowhawk. According to Greek mythology Nisus, the king of Megara, was turned into a sparrowhawk after his daughter, Scylla, cut off his purple lock of hair to present to her lover (and Nisus' enemy), Minos.

The Eurasian sparrowhawk forms a superspecies with the rufous-chested sparrowhawk of eastern and southern Africa, and possibly the Madagascar sparrowhawk. Geographic variation is clinal, with birds becoming larger and paler in the eastern part of the range compared to the western part. Within the species itself, six subspecies are generally recognised:

The Eurasian sparrowhawk is a small bird of prey with short, broad wings and a long tail, both adaptations to manoeuvring through trees. Females can be up to 25% larger than males and weigh up to twice as much. Marked size difference in this direction is unusual in higher vertebrates but typical in birds of prey, and most marked in birds of prey which hunt other birds.