The swallow-tailed hummingbird (Eupetomena macroura) is a species in the hummingbird family (Trochilidae), found mainly in east-central South America. Most authorities place it in the genus Eupetomena, although some place it in Campylopterus based on song and the thick shafts of the males' first primaries. Its common name and specific epithet (which means "large-tailed") both refer to the long, deeply forked, somewhat swallow-like tail.
Its plumage is brilliant iridescent green, with a blue head, upper chest, tail and vent. The tiny white spot behind the eye, common among hummingbirds, is often not visible in this species, but the white ankle tufts, also common among the Trochilinae, are well-developed. The remiges are blackish-brown. It has a slightly decurved medium-long black bill. The sexes are very similar, but females are about one-fourth smaller and slightly duller than males on average. Immature birds appear like females, but their heads are particularly dull and brownish-tinged.