The Knights Hospitaller, also known as the Hospitallers, Order of Hospitallers, Knights of St John, Order of St John, and currently The Sovereign Military and Hospitaller Order of St. John of Jerusalem, called of Rhodes, called of Malta, were among the most famous of the Western Christian military orders during the Middle Ages.

The Hospitallers arose as a group of individuals associated with an Amalfitan hospital in the Muristan district of Jerusalem, which was dedicated to St John the Baptist and was founded around 1023 by Blessed Gerard to provide care for poor, sick or injured pilgrims to the Holy Land. Note that other sources claim the Amalfitan order and Amalfitan hospital were different from the order of Gerard Thom and their hospital.

After the Western Christian conquest of Jerusalem in 1099 during the First Crusade, the organisation became a religious and military order under its own charter, and it was charged with the care and defence of the Holy Land. Following the conquest of the Holy Land by Islamic forces, the order operated from Rhodes, over which it was sovereign, and later from Malta where it administered a vassal state under the Spanish viceroy of Sicily.

The order was weakened by Napoleon's capture of Malta in 1798 and became dispersed throughout Europe. It regained strength during the early 19th century as it repurposed itself toward humanitarian and religious causes. In 1834 the order, by this time known as the Sovereign Military Order of Malta (SMOM), acquired new headquarters in Rome where it has remained since.

Five contemporary, state-recognised chivalric orders which claim modern inheritance of the Hospitaller tradition all assert that the Roman Catholic SMOM is the original order, and that four non-Catholic orders stem from the same root: Protestant orders exist in Germany, the Netherlands and Sweden, and a non-denominational British revival is headquartered in the United Kingdom.