The rheas (Rheidae) are three species of large, flightless birds native to South America. Their wings are large for a flightless bird and are spread while running, to act like sails. Unlike most birds, rheas have only three toes.

The name comes from Rhea the daughter of Uranus, the sky, and Gaia, the earth, in Greek mythology. She was known as "the mother of gods". The reason for naming the birds after Rhea is not known.