The white-flanked antwren (Myrmotherula axillaris) is a species of bird in subfamily Thamnophilinae of family Thamnophilidae, the "typical antbirds".

It is found from Honduras to Panama in Central America, in every mainland South American country except Argentina, Chile, Paraguay, and Uruguay, and on Trinidad.

[2] The white-flanked antwren was described by the French ornithologist Louis Pierre Vieillot in 1817 and given the binomial name Myrmothera axillaris.

The International Ornithological Committee (IOC) and BirdLife International's Handbook of the Birds of the World assign it these five subspecies:[2][4][5] The Clements taxonomy adds a sixth subspecies, M. a. luctuosa, that the other two systems treat as a separate species, the silvery-flanked antwren (M. luctuosa).

Adult males of the nominate subspecies M. a. axillaris have a dark gray head, neck, back, and rump with a hidden white patch between the shoulders.

Their throat and flanks are white, their breast and belly rich buff, their sides olive-brown, and their crissum reddish brown.

The subspecies are found thus:[9][10][11][12][13][14][15][7] The white-flanked antwren inhabits the understorey and mid-storey of evergreen forest including terra firme, várzea, igapó, and transitional types.

Its nest is a deep cup made of dead leaves and leaf skeletons that is lined, held together, and hung in a branch fork by fungal rhizomorphs.

That of M. a. axillaris, M. a. heterozyga, and M. a. fresnayana is "a rapid uncountable series of abrupt notes, somewhat low-pitched...dropping sharply in pitch and gaining in intensity at beginning, then more or less levelling".

It "is seemingly more tolerant of disturbance than are congeners, its numbers even apparently increasing in Amazonian forest exposed to fire".