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

The genus also includes two other sylphs, the violet-tailed (A. coelestis) and Venezuelan (A. berlepschi), and the three have several times been suggested to be either one or two species.

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.

The tail's upper surface is iridescent blue, green, and violet and the underside bluish black.

Their tail is short and somewhat forked, with dark blue outer feathers broadly tipped with white.

A. k. mocoa's upperparts are a shinier green than the nominate and a sapphire blue to purple throat.

[6] The subspecies of long-tailed sylph are found thus:[3][6] The long-tailed sylph inhabits generally open landscapes like scrublands, clearings and edges of forest, gardens, secondary forest, and high-elevation grassland.

On the Pacific side of southwestern Colombia, A. k. emmae apparently is present only in the wet season and is replaced by the violet-tailed sylph between January and April.

Males build the nest, a bulky dome of moss and plant fibers with a side entrance.

Its calls include "a repeated, short, buzzy or raspy 'dzzrt'...a more drawn-out, higher-pitched 'bzzeeew' [and] a thin high-pitched rising twittering.