We describe a new species of gnatcatcher, Polioptila clementsi, from white-sand (varillal) forest at the Allpahuayo-Mishana National Reserve, about 25 km by road west of Iquitos, Peru. To date, the new species is known only from the reserve, and is rare even there. Comparisons of morphological and vocal characters confirm that it is a member of the Polioptila guianensis complex, which comprises at least three poorly known, allopatric taxa ranging from the Guianas and the Rio Negro region through much of Amazonia south of the Amazon River. Roughly equivalent levels of phenotypic differentiation are documented for all taxa east of the Andes, including the new species. In consideration of the fact that some other species complexes in the genus comprise sister taxa showing lower levels of phenotypic differentiation, both morphologically and vocally, we recommend that Polioptila guianensis, P. facilis, and P. paraensis henceforth be recognized as separate species.