The souimanga sunbird is 10 cm long with a wingspan of 13 cm. The black bill is long, thin and curved. Males of the nominate subspecies have a metallic green head, back and throat. The breast is blackish with a more or less continuous red band while the belly is yellow and the wings and tail are brown. There are yellow tufts at the sides of the breast which become visible when the birds lift their wings in courtship display. Males presumably moult into a duller eclipse plumage by March–April, losing most of the metallic and red feathering for a few months. Females have grey-brown upperparts, a dull yellow belly and a grey throat and breast with darker markings. Juveniles are similar to the adult females but the chin and throat are sometimes black and the upperparts may be more olive.