Adults are mainly olive-green on the upperparts with white underparts; they have a red iris and a grey crown edged with black. There is a dark blackish line through the eyes and a wide white stripe just above that line. They have thick blue-grey legs and a stout bill. They are yellowish on the flanks and under their tail.
This bird, not always seen, may sing for long periods of time; it appears to be endlessly repeating the same question and answer. It holds the record for most songs given in a single day among bird species.
The three South American subspecies have a simpler song, a chestnut iris, and different remiges proportions. They are sometimes split as the Chivi Vireo, V. chivi. The three races concerned are V. o. chivi, V. o. vividior, and V. o. tobagoensis, a large form endemic to Tobago. The Yellow-green Vireo of Central America is also sometimes considered to be a subspecies of Red-eyed Vireo and called Vireo olivaceus flavoviridis by such authors.
The breeding habitat is open wooded areas across Canada and the eastern and northwestern United States; the Latin American population occurs in similar habitat and is partly resident all-year. North American birds migrate to South America. South American birds move north to Central America; also southwestwards to central and northern Argentina. This vireo is one of the more frequent American passerine vagrants to western Europe, with more than one hundred records, mainly in Ireland and Great Britain. In northern Ohio, it seems to return to breed at about the same time as one century ago; intriguingly, it might actually leave for winter quarters one or two weeks earlier at present than it did in the past.
Red-eyed Vireos glean insects from tree foliage, favouring caterpillars and aphids and sometimes hovering while foraging. They also eat berries, especially before migration and in the winter quarters where trees bearing popular fruit like Alchornea glandulosa or Gumbo-limbo (Bursera simaruba) will even attract them to parks and gardens. Fruit are typically not picked up from a hover, but the birds often quite acrobatically reach for them, even hanging upside down.
The nest is a cup in a fork of a tree branch. This bird suffers from nest parasitism by the Brown-headed Cowbird (Molothrus ater) in its northern range, and Shiny Cowbird (M. bonariensis) further south.