The Crimson Sunbird, Aethopyga siparaja, is a sunbird. The sunbirds are a group of very small Old World passerine birds which feed largely on nectar, although they will also take insects, especially when feeding young. Flight is fast and direct on their short wings. Most species can take nectar by hovering like a hummingbird, but usually perch to feed most of the time.

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 11cm 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 the Republic of Singapore.