Medium-sized among songbirds, scarlet tanagers have thick rounded bills and heads that are relatively large for their body size. Adult males have brilliant red feathers with black wings and tails. Females and nonbreeding males are mostly olive-green, but with black wings and tails. 

scarlet tanagers are about 6.3-6.7 inches (16-17 centimeters) long with a wingspan of 9.8-11.4 inches (25-29 centimeters).

In their breeding grounds, they inhabit large forested areas, woodlands, and semi-open areas like parks and gardens. In the winter, they are found in lowland forests near the Andean mountains.

During the breeding season both males and females sing, unlike the majority of songbird species. The female's song is softer and less harsh version than that of the male. When not singing, scarlet tanagers call out to each other with a "chip-burr" calls.

They eat insects and berries, often searching high among leafy trees or in low shrubs for their food. 

These birds travel in mixed-species flocks on their migration to and from the South American forests and eastern North America. In their wintering grounds, scarlet tanagers form loose flocks with several other species of birds to forage for food. Males compete for territories with other males during the breeding season. Scarlet tanagers are monogamous throughout a single breeding season, with males and females pairing up to breed and take care of young. Individuals select different mates from year to year.

Males arrive to their forested breeding grounds ahead of females, establishing their territory though song. Females build a shallow, open-cup nest made of twigs and grasses, where they lay a clutch of two to five eggs (but usually three to four) that are blue-green in color, sometimes with reddish-brown specks. Females incubate the eggs, which hatch after about two weeks. Both parents feed the nestlings which fledge the nest after a period of nine to 15 days. 

Like other species, the scarlet tanager is noted for being at particular risk of brood parasitism from the brown-headed cowbird. As a brood parasite, the brown-headed cowbird lays its eggs in the nest of a host species. The host parents treat cowbird eggs and young as their own which frequently hatch sooner. Cowbird young develop more rapidly and can push out eggs and young of the host nest as well as outcompete host young for parental feeding.