The Ostrów or Astrava Agreement was a treaty between Jogaila, King of Poland and Grand Duke of Lithuania, and his cousin Vytautas the Great, signed on 4 August 1392. The treaty ended the destructive Lithuanian Civil War, launched in 1389 by Vytautas who hoped to gain political power, and concluded the power struggle between the two cousins that erupted in 1380 after Jogaila secretly signed the Treaty of Dovydiškės with the Teutonic Knights. The Ostrów Agreement did not stop attacks from the Teutonic Knights and the territorial dispute over Samogitia continued up to 1422. According to the treaty, Vytautas became the ruler of Lithuania, but he also acknowledged Jogaila's rights to Lithuania. The details of the Polish–Lithuanian relationship were clarified in several later treaties, including the Union of Vilnius and Radom in 1401 and Union of Horodło in 1413.

Jogaila, later Władysław II Jagiełło, was Grand Duke of Lithuania, later giving the position to his cousin Vytautas in exchange for the title of Supreme Duke of Lithuania (1401–1434) and then King of Poland (1386–1434), first alongside his wife Jadwiga until 1399, and then sole ruler of Poland. Born a pagan, he converted to Catholicism in 1386 and was baptized as Ladislaus in Kraków, married the young Queen Jadwiga, and was crowned King of Poland as Władysław II Jagiełło. In 1387, he converted Lithuania to Catholicism. His own reign in Poland started in 1399, upon the death of Queen Jadwiga, lasted a further thirty-five years, and laid the foundation for the centuries-long Polish–Lithuanian union. He was a member of the Jagiellonian dynasty in Poland that bears his name and was previously also known as the Gediminid dynasty in the Grand Duchy of Lithuania. The dynasty ruled both states until 1572, and became one of the most influential dynasties in late medieval and early modern Europe.