The ruby-crowned kinglet is a very small passerine bird found throughout North America. It is a member of the kinglet family. The bird has olive-green plumage with two white wing bars and a white eye-ring. Males have a red crown patch, which is usually concealed. The sexes are identical, and juveniles are similar in plumage to adults. It is one of the smallest songbirds in North America. The ruby-crowned kinglet is not closely related to other kinglets and was moved from Regulus to its own genus, Corthylio, in 2021. Three subspecies are currently recognized.

A kinglet is a small bird in the family Regulidae. Species in this family were formerly classified with the Old World warblers. "Regulidae" is derived from the Latin word regulus for "petty king" or prince, and refers to the coloured crowns of adult birds. This family has representatives in North America and Eurasia. There are six species in this family; one, the Madeira firecrest, Regulus madeirensis, was only recently split from common firecrest as a separate species. One species, the ruby-crowned kinglet, differs sufficiently in its voice and plumage to be afforded its own genus, Corthylio.