Union of Lublin, (1569), pact between Poland and Lithuania that united the two countries into a single state.

After 1385 (in the Union of Krewo) the two countries had been under the same sovereign. But Sigismund II Augustus had no heirs; and the Poles, fearing that when he died the personal union between Poland and Lithuania would be broken, urged that a more complete union be formed. After the Livonian War began (1558) and Muscovy presented a serious threat to Lithuania, many of the Lithuanian gentry also desired a closer union with Poland and in 1562 made a proposal for merging the two states.

The dominant Lithuanian magnates, however, feared that a merger would diminish their power and blocked the proposal as well as subsequent initiatives. When representatives from both countries at a meeting of the Sejm (legislature) at Lublin (January 1569) failed to reach an accord, Sigismund II annexed the Lithuanian provinces of Podlasie and Volhynia(including the regions of Kiev and Bracław), which together constituted over one-third of Lithuania’s territory.

Although the Lithuanian magnates wanted to oppose Poland, the gentry declined to enter a new war, forcing negotiations for forming a union to be resumed in June.

On July 1, 1569, the Union of Lublin was concluded, uniting Poland and Lithuania into a single, federated state, which was to be ruled by a single, jointly selected sovereign. Formally, Poland and Lithuania were to be distinct, equal components of the federation, each retaining its own army, treasury, civil administration, and laws; the two nations agreed to cooperate with each other on foreign policy and to participate in a joint Diet. But Poland, which retained possession of the Lithuanian lands it had seized, had greater representation in the Diet and became the dominant partner. The Polish–Lithuanian state remained a major political entity until it was partitioned toward the end of the 18th century.