In astrophysics, Theia is the name of a hypothetical small planet that, according to one theory of the Moon's formation, collided with the Earth over four billion years ago. This impact destroyed Theia and threw massive quantities of ejecta into orbit around Earth, from which the Moon coalesced in a matter of months or days. Earth also gained significant amounts of angular momentum from the collision, as well as increasing its total mass to its modern level. See Giant impact theory.

According to this theory, Theia would have been approximately Mars-sized and struck Earth at a glancing angle. Little else is known about it. It was named after the mythological Greek Titan Theia, who gave birth to the Moon goddess Selene.