Algirdas (c. 1296 – May 1377) is a monarch of medieval Lithuania.

Algirdas ruled the Grand Duchy of Lithuania from 1345 to 1377, which chiefly means monarch of Lithuanians and Ruthenians.

With the help of his brother Kęstutis, who defends the western border of the Duchy, he creates a vast empire stretching from the Baltics to the Black Sea and reaching within fifty miles of Moscow.