In Roman mythology, Sancus was the god of loyalty, honesty, and oaths. His cult is one of the most ancient of Romans, probably derived from Etruscan or Oscan influences.

He was also the god who protected loyalty in commerce, and contracts in particular. Some forms of swearings were used in his name and honour at the moment of the signing of contracts and other important civil acts. Some words (like sanction - for the case of disrespect of pacts) have their etymology in the name of this god, whose name is connected with sancire "to hallow" (hence sanctus, "hallowed").

There was a temple to him on the Quirinal Hill, under the name Semo Sancus Dius Fidus.