The Harnen was a river in the south of Gondor.
Course
The approximately 500 mile course of the Harnen began in the southern slopes of the Ephel Dúath, it flowed south-west for about 250 miles, after that it bent west and then sped for another 250 miles to the Belegaer, which it entered in a wide delta by the Bay of Belfalas. The mouth of the Harnen is located about halfway between the mouths of the Anduin and Umbar. The Harad Road crosses the Harnen about 65 miles upstream from the point where the river turns west.
In the year T.A. 1050, Ciryaher crossed the Harnen with his army and defeated the Men of the Harad. Thereafter their kings acknowledged the overlordship of Gondor and their sons lived as hostages in the court of the king of Gondor. In the time of King Ciryaher the realm of Gondor extended south to the river Harnen and beyond it along the coast to the peninsula and haven of Umbar.
After the wars of the Kin-strife, Castamir's sons and their followers sailed from Pelargir in the year T.A. 1448 and established an independent realm in Umbar, which had been a part of Gondor since its conquest by Eärnil I in the year T.A. 933. After that date the river formed the southern border of the fief South Gondor, which was contested between Gondor and the Corsairs of Umbar. It is possible that the northern portion of the river Harnen upstream from the point where the Harad road crossed the river Harnen was the border between Gondor and Near Harad.