The Sacred Military Constantinian Order of Saint George is a religious order of knighthood and the head of the Royal House of Bourbon Two Sicilies, the dynasty which ruled over southern Italy from 1734 to 1861. The current head of the Royal House is His Royal Highness Prince Charles of Bourbon Two Sicilies, Duke of Castro who, together with his wife, Her Royal Highness Princess Camilla of Bourbon Two Sicilies, Duchess of Castro, is very active in promoting the cultural, artistic, historical and spiritual identity of southern Italy.

The order was founded by King Afonso I to honour a group of knights of the Order of Saint James of the Sword from the Kingdom of León who assisted him in retaking Santarém from the Moors on the Feast of Saint Michael, May 8, 1147. Originally, the order was formed from members of the Military Order of Saint James. The name was chosen in honour of the military saint archangel Michael, who assisted in the victory of the Conquest of Santarém from the Moors on 8 May 1147, in the shape of a wing in the sky.