The Crusader states, also known as Outremer, were four Catholic realms in the Middle East that lasted from 1098 to 1291. These feudal polities were created by the Latin Catholic leaders of the First Crusade through conquest and political intrigue. The four states were the County of Edessa (1098–1150), the Principality of Antioch (1098–1287), the County of Tripoli (1102–1289), and the Kingdom of Jerusalem (1099–1291). The Kingdom of Jerusalem covered what is now Israel and Palestine, the West Bank, the Gaza Strip, and adjacent areas. The other northern states spanned the coastal areas of what are now Syria, southeastern Turkey, and Lebanon. The description "Crusader states" can be misleading, as from 1130 very few of the Frankish population were crusaders. The term "Outremer", used by medieval and modern writers as a synonym, is derived from the French for overseas.

In 1098, the armed pilgrimage to Jerusalem passed through Syria. The crusader Baldwin of Boulogne replaced the Greek Orthodox ruler of Edessa after a coup d'état, and Bohemond of Taranto remained as the ruling prince in the captured city of Antioch. In 1099, Jerusalem was taken after a siege. Territorial consolidation followed, including the taking of Tripoli. At the states' largest extent, their territory covered the coastal areas of southern modern Turkey, Syria, Lebanon, Israel, and Palestine. Edessa fell to a Turkish warlord in 1144, but the other realms endured into the 13th century before falling to the Mamluk Sultanate of Egypt. Antioch was captured in 1268 and Tripoli in 1289. When Acre, the capital of the kingdom of Jerusalem, fell in 1291, the last territories were quickly lost, with the survivors fleeing to the Kingdom of Cyprus (established after the Third Crusade).

The study of the crusader states in their own right, as opposed to being a sub-topic of the Crusades, began in 19th-century France as an analogy to the French colonial experience in the Levant. 20th-century historians rejected this. Their consensus view was that the Franks, as the western Europeans were known, lived as a minority society that was largely urban, isolated from the indigenous peoples, with separate legal and religious systems. The indigenous peoples were from Christian and Islamic traditions speaking Arabic, Greek, and Syriac.