The violet-tailed sylph (Aglaiocercus coelestis) is a species of hummingbird in the "coquettes", tribe Lesbiini of subfamily Lesbiinae.

Several additional species have been proposed for inclusion but they have almost conclusively been shown to be hybrids with kingii or one of the other sylphs.

Nominate females have a glittering blue crown and a whitish spot behind the eye; otherwise their upperparts are similar to the male's.

Their tail is short and unforked, bluish green with white tips on the outer feathers.

[6] Males of subspecies A. c. aethereus have a green gorget and females a lightly spotted throat but both are otherwise essentially the same as the nominate.

[6] The nominate subspecies of violet-tailed sylph is found from the Pacific slope of Colombia's Western Andes into north and central Ecuador.

A. c. aethereus is found on the Andes' Pacific slope in southwestern Ecuador between El Oro and Loja provinces.

[6][7] The violet-tailed sylph is generally sedentary but makes some seasonal elevational movements to follow flowering events.