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.