Kęstutis (Belarusian: Кейстут; Lithuanian pronunciation: [kæsˈtuːtɪs]; born ca. 1297, died on August 3 or August 15, 1382 in Kreva) was monarch of medieval Lithuania. He was the Duke of Trakai and governed the Grand Duchy of Lithuania, 1342–82, together with his brother Algirdas (until 1377), and with his nephew Jogaila (until 1381). He ruled over the Lithuanians and Ruthenians.

The name "Kęstutis" is a derivative from the old form of the name Kęstas, which is a shortened version of such Lithuanian names as Kęstaras, Kęstautas (there kęs-ti means to cope). Historic writing sources reflect different Lithuanian pronunciation.

Kęstutis was the son of the Grand Duke Gediminas. His younger brother, Jaunutis, succeeded his father as Grand Duke of Lithuania. Together with his brother Algirdas, Kestutis conspired to remove Jaunutis from power. They were successful in their efforts. They divided their holdings into an eastern and western sphere of influence. The Duchy of Trakai was established in 1337 as a result. Kęstutis' efforts were concentrated in the west, while Algirdas' were concentrated in the eastern part of these territories. Kęstutis organized the defence of western Lithuania and Samogitia against the Teutonic Knights, and organized raids against the German Order.