Kęstutis (born c. 1300—died Aug. 15, 1382, Kriavas, Lithuania?) was the grand duke of Lithuania (1381–82) who defended his country’s western borders against the Teutonic Knights.
Kęstutis was one of the seven sons of Gediminas, the grand duke of Lithuania (reigned 1316–41), who had built that nation into a powerful east European empire. Kęstutis fought to make his brother Algirdas (Olgierd) the official grand duke of Lithuania (1345), with himself as coruler. Thereafter, while Algirdas extended Lithuania’s territory to the south and east, Kęstutis prevented its neighbours, the Teutonic Knights, from conquering Lithuania and making it the centre of their eastern Baltic domain. Having unsuccessfully attempted in 1345 to occupy Samogitia (the land corresponding to most of present-day Lithuania), the Knights later resorted to a tactic of continual raids on Lithuanian territory. In response, Kęstutis led expeditions into their Prussian lands, and in 1370 he and Algirdas attacked the order, imposing heavy losses upon it at the Battle of Rudava. In the same year, after a prolonged dispute (1349–70) with Poland over Galicia and Volhynia, Kęstutis gained control over both regions.
After Algirdas’ death (1377) Kęstutis became embroiled in a series of dynastic intrigues. Although Algirdas was succeeded by his son Jogaila (Jagiełło), the harmonious relationship that had existed between Kęstutis and the Grand Duke disappeared. Jogaila’s brother Andrew sought alliances with the Livonian Order (a branch of the Teutonic Order) and with Muscovy against Kęstutis, while another brother, Skirgaila (Skirgiełło), plotted against Kęstutis with the Teutonic Knights. These circumstances forced Kęstutis to sign a 10-year peace treaty with the Teutonic Order (1379). The following year Jogaila also concluded a secret treaty with the order against Kęstutis. Informed of his nephew’s act by the Knights, Kęstutis arrested Jogaila and became grand duke of Lithuania (1381). But while Kęstutis was suppressing a rebellion in his Russian territories beyond the Dnieper River in 1382, Jogaila escaped and seized the major Lithuanian cities of Vilnius and Trakai. When Kęstutis arrived at Trakai, he was arrested and brought to Kriavas, where he was presumably killed. Four years later (1386) Jogaila became king of Poland as Władysław II Jagiełło.