The white-flanked antwren (Myrmotherula axillaris) is an insectivorous bird in the antbird family, Thamnophilidae.
The white-flanked antwren is typically 10.7 cm long, and weighs 8.1 g. The adult male has dark grey upperparts, black underparts, and black wings with bars of white spots. The flanks and underwings are white. The female and immature male have brown upperparts, yellowish-buff underparts and weakly barred rufous wings. Her flanks and underwings are white, much like the male.
Males of the distinctive western race M. a. melaena have black upperparts and underparts, and the female is darker than nominate M. a. axillaris.