Oultrejordain (French for "beyond the Jordan") was the name used during the Crusades for an extensive and partly undefined region to the east of the Jordan river, an area known in ancient times as Edom and Moab.

Oultrejordain extended southwards through the Negev Desert to the Gulf of Aqaba. To the north and east (the ancient Gilead) there were no real borders - to the north was the Dead Sea and to the east were caravan and pilgrimage routes, part of the Muslim Hijaz. These areas were also under the control of the sultan of Damascus, and by custom the two opponents rarely met there, for battle or for other purposes.

Before the First Crusade Oultrejordain was controlled by the Fatimids of Egypt, who withdrew when the Crusaders arrived. The various tribes there quickly made peace with the Crusaders. The first expedition to the area was under Baldwin I of Jerusalem, who built the castle of Montreal in 1115. The crusaders also controlled the area around Petra, where they set up an archbishopric under the authority of the Latin Patriarch of Jerusalem.

There were very few Christians in Oultrejordain, most of the inhabitants being Shiite Bedouin nomads. Many of the Syrian Christians who lived there were transplanted to Jerusalem in 1115 to fill up the former Jewish quarter (the Jews had been either killed or expelled). The other Christians who lived in Oultrejordain were nomadic or semi-nomadic and were often distrusted by the Crusaders.

A lordship was established there after the expedition of Baldwin I, but due to the relative size and inaccessibility of the area, the lords of Oultrejordain tended to claim some independence from the kingdom. There was also a tradition that the lord of Oultrejordain could not hold any other positions in the kingdom at the same time, so the lords were somewhat cut off from political life. Around 1134 there was a revolt against King Fulk under Hugh II of Le Puiset and Roman of Le Puy, who were defeated and exiled. In 1142, Fulk built the castle of Kerak, replacing Montreal as the Crusader stronghold in the area. Other castles in Oultrejordain included Safed, Toron, and Subeibe.

In 1147 the lord of Oultrejordain was involved in the decision to attack Damascus during the Second Crusade, despite the truce between Jerusalem and Damascus that was vital to the survival of the kingdom and especially the lordship. The crusade ended in defeat and the security of the lordship diminished as a result.

When Raynald of Chatillon became lord in 1177, he began to claim that the king had no authority in Oultrejordain and acted as a petty ruler. He used his position to attack pilgrims and caravans, and threatened to attack Mecca, which resulted in an invasion of the kingdom by Saladin in 1187. By 1189 Saladin had taken all of Oultrejordain and destroyed its castles. In 1229 it was briefly recovered by treaty by Frederick II, Holy Roman Emperor, but the remnant of the kingdom never again controlled territory to the east of the Jordan.

While under Crusader control, the Bedouin nomads were generally left to themselves, although the king collected taxes on caravans passing through. The land was relatively good for agriculture, and wheat, pomegranates and olives were grown there. Salt was also collected from the Dead Sea.

Oultrejordain was also known in Latin as Transjordan, and covered territory that would later become part of the Emirate of Transjordan and the modern country of Jordan.