The Red-billed Tropicbird, Phaethon aethereus, also known as the Boatswain Bird is a tropicbird, one of three closely related seabirds of tropical oceans. It occurs in the tropical Atlantic, eastern Pacific and Indian Oceans.

The Indian Ocean race of the Red-billed Tropicbird, P. a. indicus, was at one time considered a full species, the Lesser Red-billed Tropicbird.

The Red-billed Tropicbird breeds on tropical islands laying a single egg directly onto the ground or a cliff ledge. It disperses widely when not breeding, and sometimes wanders far, including an amazing record from Great Britain. One has recently been found in eastern Nova Scotia, Canada. They feed on fish and squid, but are poor swimmers.

The adult is a slender mainly white bird, 48cm long excluding the very long central tail feathers, which double the total length. The wingspan is about 1 metre, and the long wings have black markings on the flight feathers. There is black through the eye. The bill is red.

Sexes are similar, although males average longer tailed, but juveniles lack the tail streamers, are greyer-backed, and have a yellow bill.

P. a. indicus has a reduced black eyestripe, and a more orange-tinted bill.