The Constitution of 3 May 1791, titled the Government Act, was a written constitution for the Polish–Lithuanian Commonwealth adopted by the Great Sejm that met between 1788 and 1792. The Commonwealth was a dual monarchy comprising the Crown of the Kingdom of Poland and the Grand Duchy of Lithuania; the new constitution was intended to address political issues following a period of political agitation and gradual reform that began with the Convocation Sejm of 1764 and the election that year of Stanisław August Poniatowski, the Commonwealth's last king. It is the third written constitution in history, after that of the United States and San Marino.

The Polish–Lithuanian Commonwealth, formally known as the Kingdom of Poland and the Grand Duchy of Lithuania, or simply Poland–Lithuania, was a bi-confederal state, sometimes called a federation, of Poland and Lithuania ruled by a common monarch in real union, who was both King of Poland and Grand Duke of Lithuania. It was one of the largest and most populous countries of 16th- to 17th-century Europe. At its largest territorial extent, in the early 17th century, the Commonwealth covered almost 1,000,000 km2 (400,000 sq mi) and as of 1618 sustained a multi-ethnic population of almost 12 million. Polish and Latin were the two co-official languages, and Roman Catholicism served as the state religion.