They have medium-length thin down-curved bills and brush-tipped tubular tongues, both adaptations to their nectar feeding.

The adult male has a crimson breast and maroon back, with black malar stripes.

The female has an olive-green back, yellowish breast and white tips to the outer tail feathers.

In most of the range, males have a long green-blue tail, but A.s. nicobarica of the Nicobar Islands and the former subspecies A. vigorsii (Western crimson sunbird) of the Western Ghats of India lack the long central tail feathers.

The crimson sunbird is a resident breeder in tropical southern Asia from India, through Nepal, Bangladesh and Myanmar to Indonesia and Brunei.