The ruby-crowned kinglet was formally described in 1766 by the Swedish naturalist Carl Linnaeus in the twelfth edition of his Systema Naturae under the binomial name Motacilla calendula.

[3] Linnaeus based his description on "The Ruby-crowned wren" that had been described and illustrated in 1758 by English naturalist George Edwards in his Gleanings of Natural History.

[4] The French zoologist Mathurin Jacques Brisson in 1760 had also published a description based on Edwards and had coined the Latin name Calendula Pensilvanica.

[5] Although Brisson coined Latin names, these do not conform to the binomial system and are not recognised by the International Commission on Zoological Nomenclature.

[13] Phylogenetic evidence indicates that the ruby-crowned kinglet's lineage diverged from the rest of Regulus during the mid- to late Miocene, about 10–15 million years ago.

The kinglet has a relatively plain face and head, although the male has a scarlet-red crown patch, which is usually concealed by the surrounding feathers.

[18] The kinglet usually moves along branches or through foliage with short hops, and flies with bursts of rapid wing beats.

[23] The subspecies C. c. grinnellii, breeding from southeastern Alaska to British Columbia differs significantly from the nominate calendula, and so is considered to represent a valid subspecies: it is smaller and shorter-winged, its upperparts are darker and greener, its underparts are buffy rather than grayish-olive, and the vent is tinged yellow rather than dull whitish-olive.

[24] Their breeding habitat is coniferous forests across Canada, Alaska, northern New England and the western United States.

They nest in a well-concealed hanging cup suspended from a conifer branch and may lay as many as twelve eggs in a clutch.

Often, they perform a "broken-wing" act to draw predators away from their nest, which they will defend fearlessly, mobbing the intruder which may be a cat, squirrel, or human.