Crimson-backed sunbirds are tiny, even by sunbird standards, and are only 8 cm long. They have medium-length thin down-curved bills and brush-tipped tubular tongues, both adaptations for nectar feeding.
The adult male is velvety red on the mantle and wing coverts and there is a broad red breast band. The crown is shiny green and there are pink-violet patches on the throat and rump. The underside from the breast below is yellowish. There is a black edge to the bib that separates the yellow of the underside. The larger purple-rumped sunbird can appear very similar but this sunbird has a darker maroon on the upper side while the flanks and vent are whitish. The eclipse plumage (non-breeding) of the male has more olive on the head and velvet red is restricted to the lower mantle and wing coverts. The female is olive-brown but the rump is distinctly red. They may be found in good numbers in flower-rich gardens at the edges of forests or plantations.