The Mouth of Truth (Italian: Bocca della Verità [ˈbokka della veriˈta]) is an ancient Roman marble mask in Rome, Italy, which stands against the left wall of the portico of the Santa Maria in Cosmedin church, at the Piazza della Bocca della Verità, the site of the ancient Forum Boarium (the ancient cattle market). According to an enduring medieval legend, it will bite off the hand of any liar who places their hand in its mouth, or, alternatively, any who utters a lie while their hand is in the mouth. It still attracts many visitors who insert their hands.

The massive marble mask weighs about 1,300 kg (2,900 lb) and probably depicts the face of the sea titan god Oceanus. The eyes, nostrils and mouth are open. Historians are not quite certain what the original purpose of the disc was. It was possibly used as a drain cover in the nearby Temple of Hercules Victor, which had an oculus (a round open space in the middle of the roof) similar to that of the Pantheon. Hence, it could rain inside. It is also thought that cattle merchants used it to drain the blood of cattle sacrificed to the demi-god Hercules.

In the 13th century the disc was probably removed from the temple and placed against the wall of the Santa Maria in Cosmedin. In the 17th century it eventually moved to its current location inside the portico of the church.