The king or queen of Jerusalem was the supreme ruler of the Kingdom of Jerusalem, a Crusader state founded in Jerusalem by the Latin Catholic leaders of the First Crusade, when the city was conquered in 1099. Most of them were men, but there were also five queens regnant of Jerusalem, either reigning alone suo jure ("in her own right"), or as co-rulers of husbands who reigned as kings of Jerusalem jure uxoris ("by right of his wife").

Godfrey of Bouillon, the first ruler of the Kingdom of Jerusalem, refused the title of king choosing instead the title Advocatus Sancti Sepulchri, that is Advocate or Defender of the Church of the Holy Sepulchre. In 1100 Baldwin I, Godfrey's successor, was the first ruler crowned as king. The crusaders in Jerusalem were conquered in 1187, but their Kingdom of Jerusalem survived, moving the capital to Acre in 1191. Crusaders re-captured the city of Jerusalem in the Sixth Crusade, during 1229–1239 and 1241–1244.

The Kingdom of Jerusalem was finally dissolved with the fall of Acre and the end of the Crusades in the Holy Land in 1291.

Even after the Crusader States ceased to exist, the title of "King of Jerusalem" was claimed by a number of European noble houses descended from the kings of Cyprus or the kings of Naples, and is claimed by the current king of Spain. ^{[citation needed]}

The Kingdom of Jerusalem had its origins in the First Crusade, when proposals to govern the city as an ecclesiastical state were rejected. In 1099 Godfrey of Bouillon was elected as the first Latin ruler of Jerusalem and was inaugurated in the Church of the Nativity in Bethlehem. He took the title of prince and Advocatus Sancti Sepulchri, meaning Advocate of the Church of the Holy Sepulchre. This was probably in response to the opinion that only Christ could wear a crown in Jerusalem. Advocatus was a title with which Godfrey was already familiar as the term was much used in the lands where the Crusaders originated; it referred to a layman who protected and administered Church estates. The following year, Godfrey died. His brother Baldwin I was the first to use the title of king and the first to be crowned king in the Church of the Holy Sepulchre in Jerusalem itself.

The kingship of Jerusalem was partially elective and partially hereditary. During the height of the kingdom in the mid-12th century there was a royal family and a relatively clear line of succession. Nevertheless, the king was elected, or at least recognized, by the Haute Cour. Here the king was considered a primus inter pares (first among equals), and in his absence his duties were performed by his seneschal.

The purpose-built royal palace used from the 1160s onwards was located south of Jerusalem's citadel. The Kingdom of Jerusalem introduced French feudal structures to the Levant. The king personally held several fiefs incorporated into the royal domain, that varied from king to king. He was also responsible for leading the kingdom into battle, although this duty could be passed to a constable.

While several contemporary European states were moving towards centralized monarchies, the king of Jerusalem was continually losing power to the strongest of his barons. This was partially due to the young age of many of the kings, and the frequency of regents from the ranks of the nobles.