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 often concealed. The sexes are identical apart from the crown, and juveniles are similar in plumage to adult females. 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.