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.

The swallow-tailed hummingbird was formally described in 1788 by the German naturalist Johann Friedrich Gmelin in his revised and expanded edition of Carl Linnaeus's Systema Naturae. He placed it with all the other hummingbirds in the genus Trochilus, coined the binomial name Trochilus macrourus and specified the type locality as Jamaica. Gmelin cited earlier authors including Hans Sloane who in 1725 had described and illustrated a humming bird from Jamaica and Mathurin Jacques Brisson who in 1760, had described and illustrated a hummingbird from Cayenne, French Guiana. In 1929 Carl Eduard Hellmayr argued that Gmelin conflated two different species and that his diagnosis applied to Brisson's specimen from Cayenne; Sloane's Jamaican bird was probably the red-billed streamertail. Hellmayr therefore redesignated the type locality as Cayenne. The swallow-tailed hummingbird is now placed together with the sombre hummingbird in the genus Eupetomena that was introduced in 1853 by the English ornithologist John Gould. The genus name combines the Ancient Greek eu meaning "good" and petomenos meaning "always on the wing" or "flying" (from petomai "to fly"). The specific epithet macroura is from the Ancient Greek makros meaning "long" and -ouros meaning "-tailed".

Five subspecies are recognised, the most recent was described in 1988.

With a total length of 15–17 cm (6–6+1⁄2 in), nearly half of which is made up by the tail, and weighing up to 9 g (0.32 oz), this is a relatively large hummingbird. Indeed, in much of its range it is the largest species of typical hummingbird. Its wings are also nearly 8 cm long – quite much for its size by hummingbird standards – though its bill is only of mediocre length, with c. 21 mm (0.83 in) not longer in absolute terms than that of many smaller relatives.

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.

The subspecies vary mainly in the hue of the plumage, with the blue sections ranging from green-tinged blue over ultramarine to deep royal blue, and the green sections ranging from golden bronzy-green over deep bottle-green to blue-tinged green. The nominate subspecies and E. m. simoni occur over a wide range, while the others are more localized endemics. For E. m. hirundo the blue quite dull and the tail less deeply forked. E. m. simoni is the bluest subspecies; the blue parts are dark royal blue, the green parts blue-tinged. E. m. bolivianus is the greenest subspecies; the head is more green than blue and the green parts pure bright green. E. m. cyanoviridis is another very green subspecies with the blue parts green-tinged and the green parts golden bronzy green.

Its voice includes relatively loud psek notes and weaker twitters. A tik call is given when excited or alarmed.

The swallow-tailed hummingbird is virtually unmistakable, although occasionally confused with the male violet-capped woodnymph (Thalurania glaucopis). These have only a blue cap however, the remainder of their head is the same green as the belly.