Crimson Sunbird is a resident breeder in tropical southern Asia from India to Indonesia and the Philippines. Two eggs or three eggs are laid in a suspended nest in a tree. This species is found in forest and cultivation.
Crimson Sunbirds are tiny, only 11 cm long. 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. The rump is yellow and the belly is olive. 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 call is chee-cheewee.
The Crimson Sunbird is the national bird of Singapore.