[4][5] The plain-bellied emerald was formally described in 1788 by the German naturalist Johann Friedrich Gmelin in his revised and expanded edition of Carl Linnaeus's Systema Naturae.

[6] The species had been described and illustrated by the French naturalists Mathurin Jacques Brisson in 1760 and Georges-Louis Leclerc, Comte de Buffon in 1779.

[7][8][9] Gmelin did not specify a type locality but this was designated as Cayenne, French Guiana, by Hans von Berlepsch in 1908.

In the revised classification to create monophyletic genera, the plain-bellied emerald was moved by most taxonomic systems to Chrysuronia.

Adults have a short, straight to slightly decurved, blackish bill with a red base to the mandible.

Adult females are similar to the male but have greenish spots on the sides of the throat and grayish green tips to the tail feathers.

[17] The plain-bellied emerald forages for nectar at a variety of flowering plants, shrubs, and trees; species in at least eleven families are known to be sources.

[17] The plain-bellied emerald's breeding season in the Guianas includes July and August, and it spans from October to February in northeastern Brazil.