The Order of Knights of the Hospital of Saint John of Jerusalem, commonly known as the Knights Hospitaller (), is a Catholic military order. It was founded in the crusader Kingdom of Jerusalem in the 12th century and had its headquarters there, in Jerusalem and Acre, until 1291, thereafter being based in Kolossi Castle in Cyprus (1302–1310), the island of Rhodes (1310–1522), Malta (1530–1798), and Saint Petersburg (1799–1801).
The Hospitallers arose in the early 12th century at the height of the Cluniac movement, a reformist movement within the Benedictine monastic order that sought to strengthen religious devotion and charity for the poor. Earlier in the 11th century, merchants from Amalfi founded a hospital in Jerusalem dedicated to John the Baptist where Benedictine monks cared for sick, poor, or injured Christian pilgrims to the Holy Land. Blessed Gerard, a lay brother of the Benedictine order, became its head when it was established. After the Christian conquest of Jerusalem in 1099 during the First Crusade, the Hospitallers rose in prominence and were recognized as a distinct order by Pope Paschal II in 1113.