The Order of Saint Lazarus of Jerusalem, also known as the Leper Brothers of Jerusalem or simply as Lazarists, was a Catholic military order founded by Crusaders around 1119 at a leper hospital in Jerusalem, Kingdom of Jerusalem, whose care became its original purpose, named after its patron saint, Lazarus. It was recognised by King Fulk in 1142 and canonically recognised as a hospitaller and military order of chivalry under the rule of Saint Augustine in the Papal bull Cum a Nobis Petitur of Pope Alexander IV in 1255. Although they were centred on their charism of caring for those afflicted with leprosy, the knights of the Order of Saint Lazarus notably fought in the Siege of Acre in 1191, Arsuf, and in the 1192 Battle of Jaffa. Battle of La Forbie in 1244 and in the Defense of Acre in 1291. The titular seat was successively situated at Jerusalem, then Acre. After the fall of the Kingdom of Jerusalem, the order split into two main branches – in Italy and in France.

A military order is a Christian religious society of knights. The original military orders were the Knights Templar, the Knights Hospitaller, the Order of Saint James, the Order of Calatrava, and the Teutonic Knights. They arose in the Middle Ages in association with the Crusades, both in the Holy Land, the Baltics, and the Iberian peninsula; their members being dedicated to the protection of pilgrims and Christians, as well as the defence of the Crusader states. They are the predecessors of chivalric orders.