The County of Edessa (Latin: Comitatus Edessanus) was a 12th-century Crusader state in Upper Mesopotamia. Its seat was the city of Edessa (modern Şanlıurfa, Turkey).
In the late Byzantine period, Edessa became the centre of intellectual life within the Syriac Orthodox Church. As such it also became the centre for the translation of Ancient Greek philosophy into Syriac, which provided a stepping stone for the subsequent translations into Arabic. When the Crusades arrived, it was still important enough to tempt a side-expedition after the siege of Antioch.
Baldwin of Boulogne, the first Count of Edessa, became King of Jerusalem, and subsequent counts were his cousins. Unlike the other Crusader states, the county was landlocked. It was remote from the other states and was not on particularly good terms with its closest neighbor, the Principality of Antioch. Half of the county, including its capital, was located east of the Euphrates, far to the east, rendering it particularly vulnerable. The west part of the Euphrates was controlled from the stronghold of Turbessel. The eastern border of Edessa was the Tigris, but the County may not have extended quite that far.
The fall of Edessa in 1144 was the first major setback for Outremer and provoked the Second Crusade. All the later Crusades, however, were troubled by strategic uncertainties and disagreements. The Second Crusade did not even try to recover Edessa, calculating it to be strategically better to take Damascus. But the campaign failed and Edessa was lost for the Christians.
In 1098, Baldwin of Boulogne left the main Crusading army, which was travelling south towards Antioch and Jerusalem. He went first south into Cilicia, then east to Edessa, where he convinced its lord, Thoros, to adopt him as son and heir. He also married Thoros' daughter, Arda of Armenia, who eventually became the first queen of Jerusalem. Thoros was a Christian of Armenian origin but of Greek Orthodox religion and largely disliked by his Armenian Orthodox subjects, which led to his removal from power in March 1098.