The Thick-billed Euphonia is distributed from Costa Rica south to southern Amazonia. Despite its name, the size of the bill is of extremely limited use in the identification process. The species is mostly found below 1200 m in secondary woodland, forest borders, and scrubby clearings and gardens. It is typically encountered in pairs or small groups, like most euphonias, and often joins mixed-species flocks, especially those dominated by tanagers. Males are mainly glossy steel blue with a yellow forecrown patch that reaches to just behind the eye, and bright yellow underparts including the throat. Females are, like those of most euphonias, much duller, being olive above and yellow below.