Constitution of May 3, 1791

Constitution of 3 May 1791, by Matejko. Foreground: King Stanisław August (left) enters St John's Cathedral, in Warsaw, where deputies will swear to uphold the Constitution. Background: the Royal Castle, where the Constitution has just been adopted.

The Constitution of 3 May 1791, titled the Government Act, was a written constitution for the Polish–Lithuanian Commonwealth that was 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 questions following a period of political agitation and gradual reform that began with the Convocation Sejm of 1764 and the election that year of the Commonwealth's last monarch, Stanisław August Poniatowski. It was the first codified, modern constitution (possessing checks and balances and a tripartite separation of powers) in Europe and the second in the world, after that of the United States.