The bay-breasted warbler (Setophaga castanea) is a small species of songbird in the New World warbler family, Parulidae. It is one of thirty-four species in the diverse genus Setophaga. Like all songbirds, or passerines, the species is classified in the order Passeriformes.
Adult males in breeding plumage are primarily grayish above, with two white wing bars, dark streaks on the back, and a creamy neck patch. The face is black, and the crown, throat, and sides are a dark chestnut color. Breeding females are similar in overall pattern to the males, but are paler and duller. In the nonbreeding season, both sexes gain olive-green feathers on the back, nape, and head. The rufous on the flanks is limited and may even fade away entirely in nonbreeding females. The two white wing bars are present in all plumages.