In a strict sense, the Union of Krewo comprised a set of prenuptial promises made at Kreva Castle on 14 August 1385 by Jogaila, Grand Duke of Lithuania, in regard to his prospective marriage to the underage reigning Queen Jadwiga of Poland.

After the 1385 negotiations, Jogaila, Grand Duke of Lithuania converted to Christianity, to the underage Queen Jadwiga, and was crowned King of Poland in 1386.

The union proved a decisive moment in the histories of Poland and Lithuania; it marked the beginning of four centuries of shared history of the two polities. By 1569 the Polish–Lithuanian union had developed into a new state, the Polish–Lithuanian Commonwealth, which lasted until the Third Partition of Poland in 1795.