The violet-green swallow (Tachycineta thalassina) is a small North American passerine bird in the swallow family. These aerial insectivores are distributed along the west coast from Alaska to Mexico, extending as far east as Montana and Texas. With an appearance very similar to the tree swallow, these individuals can be identified by the white rump side-patches that appear to separate their green back and purple tail. Violet-green swallows are secondary cavity nesters, found in a number of habitats including deciduous and coniferous forest. In addition to nesting in tree holes within these habitats, they are also widely observed nesting in the cracks of large cliffs.

The distinct body form of swallows distinguishes them from other passerine birds. Their long pointed wings and slim, streamlined body evolved to catch insects while in flight; the violet-green swallow is no exception. With an average body length of 12–13 cm (4.7–5.1 in), the violet-green swallow is slightly shorter and appears more compact in flight compared to some other members of the genus Tachycineta. The violet-green swallow most closely resembles the tree swallow T. bicolor, but can be distinguished by the more extensive white on the sides of the rump, and its green-and-violet glossed colour.

The violet-green swallow gets its name from the iridescent colours on its back and rump. The species is best identified by the glossy green on the top of the head and back as well as hints of purple on the nape, rump and upper tail. Below the green back, the remainder of the wing is a greyish-bronze. Similar to many other swallows, it is white below, but differs in that the white continues onto rump side-patches that almost meet at the base of the tail. These white rump side-patches sometimes look like one continuous stripe when in flight. White also extends onto the cheeks, partially surrounding the eye. The notch in the tail of the violet-green swallow is slightly shallower than that of the tree swallow, but deeper than the relatively flat tail of the cliff swallow.

Violet-green swallows exhibit more sexual dimorphism than many other swallows, with adult males showing brighter colours than adult females. Additionally, the white cheeks are not as prominent and the head is more bronze than green on the female. The females are however variable; while most are obviously duller, some are nearly as bright as males. Juveniles are not as easily identifiable compared to adults due to their duller colours. Similar to other juvenile swallows, especially the tree swallow, the juvenile violet-green swallow is white below and greyish-brown above. However, the more gradual change in colour and visible (but slightly reduced) white cheek patches can help identify juvenile individuals of the species.

The swallows have been well studied ecologically, but only recently has their phylogeny been thoroughly researched. Swallows are passerine birds grouped within the family Hirundinidae. The family Hirundinidae is split into two subfamilies, Pseudochelidoninae, which contains two species of river martins, and Hirundininae, comprising all other martins and swallows. The 82 species within the subfamily Hirundininae are further classified into three groupings; the mud nesters, basal relicts, and core martins, with the violet-green swallow falling into the last group. Violet-green swallows are one of nine species of New World tree swallows within the genus Tachycineta. This genus is a monophyletic group and is considered the sister taxon of a group of other swallow genera restricted to the New World.

Violet-green swallows are important members of the genus Tachycineta due to their large geographic range and because they are one of the two species that breed within North America. It is generally agreed that Tachycineta is split into two clades, one in North America and one in South America, but the phylogenetic placement within the genus depends on the type of analysis used. The violet-green swallow is a member of the North American grouping, along with T. bicolor (the North American tree swallow), T. euchrysea (the golden swallow of Hispaniola and Jamaica) and T. cyaneoviridis (the Bahama swallow). A mitochondrial genome analysis has placed T. thalassina as a sister taxon of the Caribbean swallows, T. euchrysea and T. cyaneoviridis, with all three species forming a sister clade to T. bicolor. Nuclear genome analyses and a consensus of both methods have reorganized this North American clade, alternating the closest sister taxon of T. thalassina between T. cyaneoviridis and T. euchrysea. Nonetheless, the taxonomy of the violet-green swallow is well understood up to the genus level, after which some uncertainty surrounds the placement of T. thalassina within the North American clade of the genus Tachycineta.

Violet-green swallows are divided into two subspecies, T. t. thalassina and T. t. brachyptera, differing in breeding ranges and wing length. T. t. thalassina, which occupies most of the species range, has slightly longer wings; it has been known to breed as far north as Alaska. The shorter winged T. t. brachyptera breeds further south in western Mexico in Baja California and Sonora. A third subspecies T. t. lepida formerly sometimes accepted is no longer considered distinct.

Hybridization has been known to occur among the swallows, with a few cases involving violet-green swallows being reported. Breeding between violet-green swallows and both tree and barn swallows has been recorded on separate occasions, but resulting offspring were not observed. In 2000, the strange plumage of a swallow seen in British Columbia, Canada, suggested the possible successful hybridization of a violet-green and a cliff swallow. Nonetheless, these observations are rare, which accounts for the minimal research into swallow hybridization.