count of Canossa (located near Reggio nell'Emilia, Italy) and founder of the house of Attoni.

Son of Siegfried, baron of Lucca, Atto joined the army of the bishop of Reggio, who rewarded him by giving him the fief of Canossa. In 951 Atto rescued Queen Adelaide (Adelaide, Saint), widow of King Lothar II of Italy, from imprisonment on an island in Lake Garda. The queen took refuge at Canossa. After her marriage to the German king and emperor Otto I, the king named Atto count of Canossa and marquis of the region known as Canossiana or Attoniana, stretching from the Apennines near Modena to beyond the Po, near Mantua, with sovereignty over Modena, Reggio, and Ferrara. Atto enlarged the castle of Canossa and built a church, endowing it with a treasure that his great-granddaughter Matilda (Matilda of Canossa), countess of Tuscany, more than 100 years later melted to pay the troops of Pope Gregory VII, fighting against the emperor Henry IV.