In 1386, Grand Duke Jogaila of Lithuania converted to Catholicism and married Queen Jadwiga of Poland. This act enabled him to become a king of Poland himself, and he ruled as Władysław II Jagiełło until his death in 1434. The marriage established a personal Polish–Lithuanian union ruled by the Jagiellonian dynasty. The first in a series of formal "unions" was the Union of Krewo of 1385, whereby arrangements were made for the marriage of Jogaila and Jadwiga. The Polish–Lithuanian partnership brought vast areas of Ruthenia controlled by the Grand Duchy of Lithuania into Poland's sphere of influence and proved beneficial for the nationals of both countries, who coexisted and cooperated in one of the largest political entities in Europe for the next four centuries. When Queen Jadwiga died in 1399, the Kingdom of Poland fell to her husband's sole possession.

In the Baltic Sea region, Poland's struggle with the Teutonic Knights continued and culminated in the Battle of Grunwald (1410), a great victory that the Poles and Lithuanians were unable to follow up with a decisive strike against the main seat of the Teutonic Order at Malbork Castle. The Union of Horodło of 1413 further defined the evolving relationship between the Kingdom of Poland and the Grand Duchy of Lithuania.