The House Sparrow (Passer domesticus) is a species of passerine bird of the sparrow family Passeridae. It occurs naturally in most of Europe, the Mediterranean region, and much of Asia. It has also been intentionally or accidentally introduced to many parts of the world, making it the most widely distributed wild bird. It is strongly associated with human habitations, but it is not the only sparrow species found near houses. It is a small bird, with feathers mostly different shades of brown and grey.

The House Sparrow is a chunky bird, typically about 16 centimetres (6.3 in) long, ranging from 14–18 centimetres (5.5–7.1 in). It has a large rounded head, a short tail, and a stout bill. In weight, the House Sparrow generally ranges from 24–39.5 grams (0.85–1.39 oz). Weight varies by sex, with females usually smaller than males. Younger birds are smaller, males are larger during the winter, and females larger during the breeding season.Between and within subspecies, there is further variation based on latitude, altitude, climate, and other environmental factors, under biological rules such as Bergmann's rule.

The plumage of the House Sparrow is mostly different shades of grey and brown. The sexes differ, with females and juveniles mostly buff, and the male marked with bold colours. The male is duller in fresh non-breeding plumage, with buff tips on many feathers. Wear and preening expose bright markings of brown and black, including a throat and chest patch, called a "bib" or a "badge". This patch is variable in width and general size, and some scientists have suggested that patches signal social status or fitness, a hypothesis which has led to a "veritable 'cottage industry'" of studies, which have only conclusively shown that patches increase in size with age. In breeding plumage, the male's crown is grey, and it is marked with black on its throat and beneath its crown. The cheeks and underparts are pale grey. The mantle and upper back are a warm brown, broadly streaked with black, while the lower back, rump and uppertail coverts are a greyish-brown. The female has no black on head or throat, nor a grey crown and its upperparts are streaked with brown. The juvenile is deeper brown, and the white is replaced by buff; the beak is pink to dull yellow.

There is some variation in the twelve subspecies of House Sparrow. The subspecies are divided into two groups, the Oriental indicus group, and the Palaearctic domesticus group. Birds of the domesticus group have grey cheeks, while indicus group birds have white cheeks, as well as bright colouration on the crown, a smaller bill, and a longer black bib. The subspecies Passer domesticus tingitanus differs little from the nominate subspecies, except in the worn breeding plumage of the male, in which the head is speckled with black and underparts are paler. P. d. balearoibericus is slightly paler than the nominate but darker than P. d. bibilicus. P. d. bibilicus is paler than most subspecies, but has the grey cheeks of domesticus group birds. The similar P. d. persicus is paler and smaller, and P. d. niloticus is nearly identical but smaller. Of the less wide ranging indicus group subspecies, P. d. hyrcanus is larger than P. d. indicus, P. d. bactrianus is larger and paler, P. d. parkini is larger and darker with more black on the breast than any other subspecies, and P. d. hufufae is paler.

The House Sparrow can be confused with a number of other seed-eating birds, especially its relatives in the genus Passer. Many of these relatives are smaller, with an appearance that is neater or "cuter", as with the Dead Sea Sparrow. The dull-coloured female often can not be distinguished from other birds, and it is nearly identical to the females of the Spanish Sparrow and Italian Sparrow. The Eurasian Tree Sparrow is smaller and more slender with a chestnut crown and a black patch on each cheek. The male Spanish Sparrow and Italian Sparrow are distinguished by their chestnut crowns. The Sind Sparrow is smaller, with the male less black on the throat and the female usually having a distinct pale supercilium.

All of the House Sparrow's vocalisations are variations on its short and incessant chirping call. Transcribed as chirrup, tschilp, or philip, this note is made as a contact call by flocking or resting birds, or by males to proclaim nest ownership and invite pairing. In the breeding season this call becomes what is called an "ecstatic call", which is similar to a song, as it is uttered by the male at great speed. Young birds, especially in captivity, also give a true song, a warbling similar to that of the European Greenfinch. Aggressive male House Sparrows give a trilled version of their call, transcribed as "chur-chur-r-r-it-it-it-it". This call is also used by females in the breeding season, to establish dominance over males while displacing them to feed young or incubate eggs. The House Sparrow gives a nasal alarm call, the basic sound of which is transcribed as quer, and it gives a shrill "chree" call in great distress. Another House Sparrow vocalisation is what has been described as an "appeasement call", a soft quee given to inhibit aggression, usually by a mated pair. These vocalisations are not unique to the House Sparrow, but are shared with small variations by all sparrows.

The House Sparrow is part of the sparrow genus Passer, which contains about 20 species, depending on the authority. Most species in the genus are between 11 and 16 cm (4–6 in) long, dull-coloured birds with short square tails and stubby conical beaks. Mitochondrial DNA suggest that speciation in Passer occurred during the Pleistocene and earlier, while other evidence suggests speciation occurred 25,000 to 15,000 years ago. Within Passer, the House Sparrow is part of the "Palearctic black-bibbed sparrows" group and a close relative of the Mediterranean "willow sparrows".

The taxonomy of the House Sparrow and its Mediterranean relatives is highly complicated. The common type of "willow sparrow" is the Spanish Sparrow, which resembles the House Sparrow in many respects. It frequently prefers wetter habitats than the House Sparrow, and it is often colonial and nomadic. In most of the Mediterranean, one or both species occur, with some degree of hybridisation. In North Africa, the two species hybridise extensively, forming highly variable mixed populations with a full range of characters from pure House Sparrows to pure Spanish Sparrows and everything between.

In much of Italy there is a type of sparrow apparently intermediate between the House and Spanish sparrows, known as the Italian Sparrow. It resembles a hybrid between the two species, and is in other respects intermediate. Its specific status and origin are the subject of much debate. In the Alps, the Italian Sparrow intergrades over a roughly 20 km (12 mi) strip with the House Sparrow, but to the south it intergrades over the southern half of Italy and some Mediterranean islands with the Spanish Sparrow. On the Mediterranean islands of Malta, Gozo, Crete, Rhodes, and Karpathos, there are other apparently intermediate birds of unknown status.

The bird's usual English and scientific names have the same meaning. The Latin word passer, like the English word "sparrow", is a term for small active birds, coming from a root word referring to speed. The Latin word domesticus means "belonging to the house", like the common name a reference to its association with humans. The House Sparrow was scientifically described by Carl Linnaeus in the 1758 edition of his Systema Naturae, as Fringilla domestica. Later the name Fringilla came to be used only for the Chaffinch and its relatives, and House Sparrow as placed in the genus Passer created by French zoologist Mathurin Jacques Brisson in 1760. The House Sparrow is called by a number of other names, including English Sparrow, chiefly in North America; and Indian Sparrow or Indian House Sparrow, for the birds of the Indian subcontinent and Central Asia. Dialectal names include sparr, sparrer, spadger, spadgick, and philip, mainly in southern England; spug and spuggy, mainly in northern England; spur and sprig, mainly in Scotland; and spatzie or spotsie in North America.