The western house martin (Delichon urbicum), sometimes called the common house martin, northern house martin or, particularly in Europe, just house martin, is a migratory passerine bird of the swallow family which breeds in Europe, north Africa and west-central Asia; and winters in sub-Saharan Africa and southwestern Asia. It feeds on insects which are caught in flight, and it migrates to climates where flying insects are plentiful. It has a blue head and upperparts, white rump and pure white underparts, and is found in both open country and near human habitation. It is similar in appearance to the two other martin species of the genus Delichon, which are both endemic to eastern and southern Asia. It has two accepted subspecies.
Both the scientific and colloquial name of the bird are related to its use of human-made structures. It builds a closed cup nest from mud pellets under eaves or similar locations on buildings usually in colonies.
It is hunted by the Eurasian hobby (Falco subbuteo), and like other birds is affected by internal parasites and external fleas and mites. Its large range and population mean that it is not threatened globally.
The adult western house martin is 13 cm (5 in) long, with a wingspan of 26–29 cm (10–11+1⁄2 in) and a weight averaging 18.3 g (21⁄32 oz). It is steel-blue above with a white rump, and white underparts, including the underwings; even its short legs have white downy feathering. It has brown eyes and a small black bill, and its toes and exposed parts of the legs are pink. The sexes are similar, but the juvenile bird is sooty black, and some of its wing coverts and quills have white tips and edgings. D. u. lagopodum differs from the nominate race in that its white rump extends much further onto the tail, and the fork of its tail is intermediate in depth between that of D. u. urbicum and that of the Asian house martin.
The white rump and underparts of the western house martin, very noticeable in flight, prevent confusion with other widespread Palaeoarctic swallows such as the barn swallow (Hirundo rustica), sand martin (Riparia riparia) or red-rumped swallow (Cecropis daurica). In Africa, confusion with grey-rumped swallow (Pseudhirundo griseopyga) is possible, but that species has a grey rump, off-white underparts and long, deeply forked tail. The western house martin flies with a wing beat averaging 5.3 beats per second, which is faster than the wing beat of 4.4 beats per second for the barn swallow, but the flight speed of 11 m/s (36 ft/s) is typical for hirundines.

The preferred habitat of the western house martin is open country with low vegetation, such as pasture, meadows and farmland, and preferably near water, although it is also found in mountains up to at least 2,200 m (7,200 ft) elevation. It is much more urban than the barn swallow, and will nest even in city centres if the air is clean enough. It is more likely to be found near trees than other Eurasian swallows, since they provide insect food and also roosting sites. This species does not normally use the reed-bed roosts favoured by migrating barn swallows.
It uses similar open habitats on the wintering grounds, but the western house martin is less conspicuous than wintering barn swallows, tending to fly higher and be more nomadic. In the tropical parts of its wintering range, like East Africa, it appears to be mainly found in the higher areas.
Breeding birds return to Europe between April and May, and nest building starts between late March in North Africa and mid-June in Lapland. The nest is a neat closed convex cup fixed below a suitable ledge, with a narrow opening at the top. It is constructed by both sexes with mud pellets collected in their beaks, and lined with grasses, hair or other soft materials. The mud, added in successive layers, is collected from ponds, streams or puddles. House sparrows frequently attempt to take over the nest during construction, with the house martins rebuilding elsewhere if they are successful. The entrance at the top of the cup is so small once it is complete that sparrows cannot take over the nest.
The western house martin tends to breed colonially, and nests may be built in contact with each other. A colony size of less than 10 nests is typical, but there are records of colonies with thousands of nests. Four or five white eggs are usually laid, which average 1.9 cm × 1.33 cm (3⁄4 in × 17⁄32 in) in size, and weigh 1.7 g (1⁄16 oz). The female does most of the incubation, which normally lasts 14–16 days. The newly hatched chicks are altricial, and after a further 22–32 days, depending on weather, the chicks leave the nest. The fledged young stay with, and are fed by, the parents for about a week after leaving the nest. Occasionally, first-year birds from the first brood will assist in feeding the second brood.