Algirdas (Belarusian: Альгерд, Polish: Olgierd) (c. 1296 – May 1377) was a monarch of medieval Lithuania. Algirdas ruled the Grand Duchy of Lithuania from 1345 to 1377, which chiefly meant monarch of Lithuanians and Ruthenians. With the help of his brother Kęstutis, who defended the western border of the Duchy, he created a vast empire stretching from the Baltics to the Black Sea and reaching within fifty miles of Moscow.

Algirdas was one of the seven sons of the Grand Duke Gediminas. Before his death in 1341 Gediminas divided his domains, leaving the youngest son Jaunutis in possession of the capital Vilnius, with a nominal priority. With the aid of his brother Kęstutis, Algirdas drove out the incapable Jaunutis and declared himself a Grand Prince in 1345. Thirty two years of his reign (1345–1377) were devoted to the development and expansion of the Grand Duchy of Lithuania. Algirdas has managed to make it one of the greatest states in Europe and the largest in the continent.

Two factors are supposed to have contributed to achieve this result; the extraordinary political sagacity of Algirdas and the life-long devotion of his brother Kęstutis. A neat division of their dominions is illustrated by the fact, that Algirdas appears almost only in East Slavic sources, whereas the Western chronicles are aware of mostly Kęstutis. The Teutonic knights in the north and the Tatar hordes in the south were equally bent on the subjection of Lithuania, while Algirdas' eastern and western neighbors Muscovy and Poland generally were hostile competitors.