Lucy's Warbler, Vermivora luciae, is the smallest New World warbler found in North America, measuring a mere 4.25 inches in length.

It is rather nondescript compared to other wood-warblers. 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 crown patch. Juveniles are paler, with a tawny rump and buffy wingbars.

Lucy's Warblers inhabit riparian mesquite and brushy country of the southwestern United States and northwestern Mexico. Lucy's is the only warbler besides Prothonotary to nest in cavities. Habitat loss and to a lesser extent, Brown-headed Cowbird parasitism are threatening this species, and populations are diminishing throughout its breeding range. The birds migrate to western Mexico in winter.

These strictly insectivorous birds forage actively, looking for the caterpillars, beetles, and leafhoppers that compose much of their diet.

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.