The black-bellied sunbird (Cinnyris nectarinioides) is a small sunbird located primarily in Africa. Two subspecies are recognized: the Cinnyris nectarinioides erlangeri, which is smaller and found in Ethiopia, Somalia, and Kenya; and the Cinnyris nectarinioides nectarinioides, which is larger and found in Kenya and Tanzania. This bird inhabits savannah, wetlands, and terrestrial freshwater areas.

This species has an average size of 13cm for males and 10 cm for females with both weights typically ranging from 4 to 6 grams. The black-bellied sunbird feeds on nectar, insects and spiders, the latter having gained it the nickname "spiderhunter". They forage in the higher levels of trees and perch in order to feed. These birds do not migrate in different seasons of the year, and inhabit the same area year round.

The black-bellied sunbird can be characterized by a curved bill used for nectar feeding and a small frame. These birds often weigh just 4–6 grams and as grown males tend to be around 13 cm large, while females remain around 10 cm. This sunbird is sexually dimorphic with differences in plumage between males and females and slight differences between breeding and non-breeding males.

A breeding male has a golden-green head, upperparts, chin, and throat. The bases of its feathers and upper tail are black with the rest of the upper tail being a dark metallic blue. Feathers are occasionally purple or a lustrous greenish-blue at the edges, with faint blue bands stretching across the feathers. They have an orange breastband, that is redder in the erlangeri species, tufts of yellow on the pecs and slight streaks of white on their flanks. The male's bill and legs are both black. The non-breeding males are similar, differentiated only by upperparts being a more dull brown and some white-yellow underparts, also known as eclipse plumage.

Female plumage is yellow-olive with streaks of dark brown on their crown. One distinguishing feature is the streak of yellow above their eye. Their tail is similar to the male, only differentiated by white tips and edges. The middle of a female belly and its undertail are pale yellow and its beak and legs are black like their male counterparts.