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.
In terms of its physical attributes, the buff-rumped warbler is 13–13.5 cm long and weighs 14.5 g. The nominate subspecies M. f. fulvicauda in western Amazonia is distinguished by its dark olive-brown upper body, grey head, buff forehead stripe, and striking buff-colored rump and tail feathers.