Sejm Czteroletni (Four-Year Sejm, also known as Sejm Wielki, the Great Sejm) was a Sejm of the Polish-Lithuanian Commonwealth held in Warsaw, inaugurated in 1788. Its goal was to restore sovereignty to, and reform, the Commonwealth, politically and economically. Its greatest achievement was adoption of the May 3rd Constitution in 1791.

The achievements of the Great Sejm were destroyed by the Targowica confederation and the intervention of the Russian Empire.

Important participants in the Great Sejm included:

• King Stanislaw August Poniatowski
• Hugo Kołłątaj
• Ignacy Potocki
• Stanisław Małachowski, Marshal of the Sejm
• Stanisław Staszic