Rhea Sylvia (also written as Rea Silvia), and also known as Ilia, was the mythical mother of the twins Romulus and Remus, who founded the city of Rome. Her story is told in the Ab Urbe Condita of Livy. According to legend, she was the daughter of Numitor, king of Albalonga and descendant of Aeneas. Numitor's brother Amulius seized the throne and killed Numitor's son. Amulius forced Rhea Sylvia to become a Vestal Virgin, a priestess to the goddess Vesta who is sworn to lifelong celibacy, so that Numitor would not have any heirs.

However, the god Mars took a fancy to Rhea Sylvia and raped her, conceiving the twins. When he learned of this, Amulius ordered Rhea Sylvia buried alive (the standard punishment for Vestal Virgins who did not remain celibate) and ordered a servant to kill the twins, but the merciful servant set them adrift in the river Tiber. The river-god, Tiberinus found the twins and gave them to a she-wolf, Lupa, to suckle, and then he rescued and married Rhea Silvia. Romulus and Remus went on to found Rome and overthrow Amulius, reinstating Numitor as King of Alba Longa.

The name Rea Silvia suggests a minor deity, a demi-goddess of forests. Silva means woods or forest, and Rea may be related to res and regnum; Rea may also be related to Greek rheô, "flow," and thus relate to her association with the spirit of the river Tiber.

See also: Rome, Romulus, Aeneas, Mars, The founding of Rome