The Great Sejm, also known as the Four-Year Sejm, is a parliament (Sejm) of the Polish-Lithuanian Commonwealth that is held in Warsaw between 1788 and 1792.

Its principal aim becomes to restore sovereignty to, and reform, the Commonwealth politically and economically.

The Sejm's great achievement is the adoption of the Constitution of May 3, 1791, often described as Europe's first modern written national constitution, and the world's second, after the United States Constitution.

The Polish Constitution is designed to redress long-standing political defects of the federative Polish-Lithuanian Commonwealth and its system of Golden Liberties.

The Constitution introduces political equality between townspeople and nobility and places the peasants under the protection of the government, thus mitigating the worst abuses of serfdom.

The Constitution abolishes pernicious parliamentary institutions such as the liberum veto, which at one time had placed a sejm at the mercy of any deputy who might choose, or be bribed by an interest or foreign power, to undo all the legislation that had been passed by that sejm.

The May 3rd Constitution seeks to supplant the existing anarchy, fostered by some of the country's reactionary magnates, with a more egalitarian and democratic constitutional monarchy.

The reforms instituted by the Great Sejm and the Constitution of May 3, 1791, wil be undone in 1792 by the Targowica Confederation and the intervention of the Russian Empire at the invitation of the Targowica Confederates.