The siege of Shaizar took place from April 28 to May 21, 1138. The allied forces of the Byzantine Empire, Principality of Antioch and County of Edessa invaded Muslim Syria. Having been repulsed from their main objective, the city of Aleppo, the combined Christian armies took a number of fortified settlements by assault and finally besieged Shaizar, the capital of the Munqidhite Emirate. The siege captured the city, but failed to take the citadel; it resulted in the Emir of Shaizar paying an indemnity and becoming the vassal of the Byzantine emperor. The forces of Zengi, the greatest Muslim prince of the region, skirmished with the allied army but it was too strong for them to risk battle. The campaign underlined the limited nature of Byzantine suzerainty over the northern Crusader states and the lack of common purpose between the Latin princes and the Byzantine emperor.

The Principality of Antioch was one of the Crusader states created during the First Crusade which included parts of modern-day Turkey and Syria. The principality was much smaller than the County of Edessa or the Kingdom of Jerusalem. It extended around the northeastern edge of the Mediterranean, bordering the County of Tripoli to the south, Edessa to the east, and the Byzantine Empire or the Kingdom of Armenia to the northwest, depending on the date.