A new species of tyrant flycatcher (Suiriri islerorum) is described from the cerrado region of Brazil and adjacent eastern Bolivia. The species previously had been confused with Suiriri suiriri affinis, with which it is syntopic at multiple sites. The new species was first identified by voice. Although cryptically similar to S. s. affinis in many respects, the new species is readily identified by all vocalizations, bill size, color pattern of the tail, and shape of the central rectrices. Most distinctive are the male–female duets, which are accompanied by dramatic wing-lifting displays not performed by any congeners. Reciprocal playback experiments of tape-recorded vocalizations demonstrated that the new species and S. s. affinis do not respond to one another's vocalizations. We provide information on the natural history of the new flycatcher, along with spectrograms of its various vocalizations. We also provide vocal analysis of all other named taxa in Suiriri, and discuss the various intrageneric relationships. In particular, S. s. affinis and S. s. bahiae, although distinct morphologically, are vocally and behaviorally similar, and respond to one another's vocalizations in playback experiments.