In the fiction of J. R. R. Tolkien, the Kin-strife was a disastrous civil war in Gondor.
The unrest that created the Kin-strife began when Valacar, the son of the Gondorian king Rómendacil II, married a woman of the Northmen of Rhovanion, Vidumavi. She bore him a son Eldacar, but many Gondorians of Númenórean blood were angered by this mixing of blood of Middle Men and Númenóreans, and the coastal provinces rebelled when Valacar grew old.
When in 1432 of the Third Age Eldacar succeeded his father the unrest grew into open rebellion, as many Gondorians saw Eldacar as a halfbreed who had no right to rule. The chief of them was his distant relative Castamir the Usurper, Lord of Ships, who in 1437 usurped the throne, forcing Eldacar into exile. During the rebellion Osgiliath was burned, and the great Dome of Stars was lost, together with its Palantír. Castamir also murdered Eldacar's son and heir Omendil. Eldacar fled to Rhovanion, and Castamir ruled in his stead.
A full decade later, in 1447, Eldacar returned with Rhovanion troops, at the same time a rebellion against Castamir's cruel rule took place. Eldacar managed to kill Castamir at the Battle of Erui, but Castamir's sons and many of their supporters fled south to Umbar. Eldacar could not follow them, as the fleet was under Castamir's control.
Not only did Gondor lose the city of Umbar for four centuries and gained a new enemy in the Corsairs of Umbar, descendants of Castamir's sons, but also many of the Númenóreans of purest blood were killed during the civil war, leaving Gondor weakened.
The Kin-strife was, along with the Great Plague, one of the chief reasons for the abandonment of the fortresses in and surrounding Mordor.