Algirdas (c. 1296 – May 1377) was Grand Duke of Lithuania from 1345 to 1377. With the help of his brother Kęstutis (who defended the western border of the Duchy), he created an empire stretching from the present Baltic states to the Black Sea and to within 80 kilometres (50 miles) of Moscow.

Algirdas was one of the seven sons of Grand Duke Gediminas. Before his death in 1341, Gediminas divided his domain, leaving his youngest son Jaunutis in possession of the capital, Vilnius. With the aid of his brother Kęstutis, Algirdas drove out the incompetent Jaunutis and declared himself Grand Duke in 1345. He devoted the next thirty-two years to the development and expansion of the Grand Duchy of Lithuania. He and Kęstutis appeared to rule jointly as co-rulers.

After becoming the ruler of Lithuania, Algirdas was titled the King of Lithuania (Latin: rex Letwinorum) in the Livonian Chronicles instead of the terms knyaz (English: prince, duke) or velikiy knyaz (grand prince).

Two factors are thought to have contributed to this result: the political sagacity of Algirdas and the devotion of Kęstutis. The division of their dominions is illustrated by the fact that Algirdas appears almost exclusively in East Slavic sources, while Western chronicles primarily describe Kęstutis. Lithuania was surrounded by enemies. The Teutonic Order in the northwest and the Golden Horde in the southeast sought Lithuanian territory, while Poland to the west and the Moscow principality to the east were generally hostile competitors.^{[citation needed]}

"The entire Rus' should belong to the Grand Duchy of Lithuania"

Algirdas held his own, also acquiring influence and territory at the expense of the Moscow principality and the Golden Horde and extending the borders of the Grand Duchy of Lithuania to the Black Sea. His principal efforts were directed toward securing the Slavic lands which were part of the former Kievan Rus'. Although Algirdas engineered the election of his son Andrius as Prince of Pskov and a powerful minority of Novgorod Republic citizens supported him against the Moscow principality, his rule in both commercial centres was (at best) precarious.

Algirdas occupied the important principalities of Smolensk and Bryansk. Although his relationship with the grand dukes of Moscow principality was generally friendly (demonstrated by his marriages to two Orthodox Russian princesses), he besieged Moscow in 1368 and 1370 during the Lithuanian–Muscovite War (1368–1372). An important feat by Algirdas was his victory over the Tatars in the Battle of Blue Waters at the Southern Bug in 1362, which resulted in the breakup of the Kipchaks and compelled the khan to establish his headquarters in the Crimea.

In a 1371 letter to Philotheus Kokkinos, the Ecumenical Patriarch of Constantinople, Algirdas titled himself as a Lithuanian King, demanded a separate metropolitan bishop (from Moscow) for Kyiv, Smolensk, Tver, Little Russia, Novosil, and Nizhny Novgorod, and denied Muscovite complaints that he attacked Moscow without a reason.