The White-bellied Woodstar is among the smallest hummingbirds like most woodstars, however it is colorful and conspicuous. The body is mostly green, with a white collar that joins a white line coming out of the eye. The gorget is metallic purple due to iridescence, so it looks dark sometimes. The black bill is long, thin and just slightly curved down. The vent and belly also show white plumage. It flies in a typical bee fashion, with smooth movements instead of the rapid bursts that larger hummingbirds do.

Like most woodstars, this species flies in a bee-like fashion, revealing a gorget that ranges from purple to black.