Lucy's warbler (Leiothlypis luciae) is a small New World warbler found in North America. This species ranges includes southwestern United States and northwestern Mexico. It is one of only two warblers to nest in cavities.
Lucy's warbler is the smallest species of New World warbler. It measures from 9 to 12 cm in length. It is rather nondescript compared to other wood-warblers, being perhaps the palest species in its family. Its head and upperparts are pale gray, while underparts are whitish. It has a white eyering and a small, pointed bill. Both sexes have a rufous rump, a diagnostic field mark. Adult males also have a small rusty patch on their crown. Juveniles are paler, with a tawny rump and buffy wingbars.
Lucy's warbler is closely related to Virginia's warbler, Nashville warbler and Colima warbler. The common name and binomial of this species commemorate Lucy Hunter Baird, daughter of ornithologist Spencer Fullerton Baird.