Saint Ansanus (Italian: Sant'Ansano) (died 304 AD), called The Baptizer or The Apostle of Siena, is the patron saint of Siena, a scion of the Anician family of Rome.

His legend states that he was born of a noble Roman family in the third century.

Ansanus openly declared his Christian faith during the persecutions of Diocletian, when he was nineteen years old.

According to tradition, St. Ansanus preached the Gospel in Bagnoregio (then Bagnorea) and the church of Santa Maria delle Carceri outside the Alban Gate was said to have been built above the prison in which he was confined.

It is also said that his own father denounced him to the authorities, but Ansanus managed to escape, and converted many at Bagnorea and later at Siena.