Exclusively Amazonian, the Yellow-bellied Dacnis principally inhabits low-lying seasonally flooded forests, and is often closely associated with waterside areas, although it ranges locally into terra firme and even secondary forests in places. The species is typically encountered in pairs, usually at mid to higher levels in trees, and is generally uncommon to scarce at most localities. The male is a striking bird clad in yellow and black, with a greenish crown, and deep red irides. The female too has red eyes, which is perhaps the most useful field mark, given that the bird is otherwise a dull greenish olive with rather paler underparts.
The name Dacnis flaviventer derives from Greek and Latin roots. The genus Dacnis is derived from the Greek word daknis which refers to an unspecified type of bird from Egypt; the specific epithet flaviventer is derived from the Latin words flavus, meaning yellow, and venter, meaning belly (Jobling 2010). In Spanish the common name is Dacnis de Vientre Amarillo (Peru), Mielero Vientre Amarillo (Venezuela), or Dacnis Ventriamarillo (Plenge 1980, Hilty 2003, Alonso et al. 2012, de Juana et al. 2012), and in Portuguese the common name is Saí-Amarela (CBRO 2010). Thus, all names refer to this species' distinctive yellow plumage.