The Sacred Military Constantinian Order of Saint George is a Roman Catholic order of chivalry. It was fictively established by Constantine the Great, though, in reality, it was founded some time between 1520 and 1545 by two brothers belonging to the Angeli Comneni family. Members of the Angeli Comneni family remained grand masters throughout the sixteenth and seventeenth centuries.
In 1699, Francesco Farnese, Duke of Parma was recognised as grand master. In 1731, his son and successor, Antonio Farnese, Duke of Parma, died without male heirs. He was succeeded by the first Bourbon grand master, Charles, Duke of Parma (later King Charles III of Spain). Since that time, members of the House of Bourbon have been grand masters of the order.
Owing to various family disputes, the grand magistracy was, until 2014 claimed by three rivals, all princely members of the House of Bourbon:
- Prince Carlo, Duke of Castro (his claim)
- Infante Carlos of Spain, Duke of Calabria (his claim - order's website)
- Prince Carlos, Duke of Parma (his claim)
There are approximately 2,800 of the Franco-Neapolitan branch and 1,800 members of the Hispano-Neapolitan branch.
The motto of the Order is In Hoc Signo Vinces ("in this sign you will conquer").