The County of Edessa (Latin: Comitatus Edessanus) was a 12th-century Crusader state in Upper Mesopotamia.

In the late Byzantine period, Edessa became the centre of intellectual life within the Syriac Orthodox Church.

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 was joined by Joscelin of Courtenay, who became lord of the fortress of Turbessel on the Euphrates, an important outpost against the Seljuk Turks.

Count Baldwin's wife had died in Marash in 1097, and after he succeeded to Edessa he married Arda, a granddaughter of the Armenian Roupenid Prince Constantine.

Bohemond's cousin Tancred became regent in Edessa (although Richard of Salerno actually governed the territory), until Baldwin and Joscelin were ransomed in 1108.

Meanwhile, Joscelin II paid little attention to the security of his county, and argued with the counts of Tripoli who then refused to come to his aid.

Joscelin continued to rule his lands west of the Euphrates, and he also managed to take advantage of the death of Zengi in September 1146 to regain and briefly hold his old capital.

His wife sold Turbessel and what was left of the county to the Byzantine Emperor Manuel I Comnenus, but these lands were conquered by Nur ad-Din and the Sultan of Rum within a year.

To the south and east were the powerful Muslim cities of Aleppo and Mosul, and the Jazira (northern Iraq).

After the sale, the wife and family of Joscelin II moved with the proceeds to the Kingdom of Jerusalem, near Acre.