The buff-rumped warbler is 13–13.5 cm long and weighs 14.5 g.  The nominate race M. f. fulvicauda of western Amazonia in Colombia, Ecuador, Peru and Brazil is dark olive-brown above with a grey head, buff supercilium, and the conspicuous rich buff rump and upper tail that give this species its English and scientific names. The lower half of the tail is blackish.
The buff-rumped warbler is 13–13.5 cm long and weighs 14.5 g. The nominate race M. f. fulvicauda of western Amazonia in Colombia, Ecuador, Peru, and Brazil is dark olive-brown above with a grey head, buff supercilium, and the distinctive rich buff rump and upper tail that give this species its English and scientific names. The lower half of the tail is blackish.