[4][5][6] The red-billed streamertail was formally described by the Swedish naturalist Carl Linnaeus in 1758 in the tenth edition of his Systema Naturae under the binomial name Trochilus polytmus.

[7] Linnaeus quoted the description in Latin by the Irish physician Patrick Browne in his The Civil and Natural History of Jamaica which had been published two years earlier in 1756.

[8] The specific epithet polytmus is from the Ancient Greek polutimos meaning "costly" or "valuable".

The male's face and most of its underparts are metallic yellowish green; the undertail coverts are blue-black or black with a bluish gloss.

[1] The red-billed streamertail forages for nectar at a wide variety of native and introduced flowering species; it especially prefers Besleria lutea.

It has also been observed "robbing" nectar from holes in flowers created by bananaquits (Coereba flaveola) and visiting wells drilled by yellow-bellied sapsuckers (Sphyrapicus varius).

Both sexes perch and repeatedly nod their heads, after which the male flies up and down in front of the female while spreading its tail streamers.

The red-billed streamertail's vocalizations include "a loud metallic ting or teet and a prolonged twink-twink-twink dropping in pitch at the end."

The whirring is synchronized with the wingbeats and video footage shows primary feather eight (P8) bending with each downstroke, creating a gap that produces the fluttering sound.