Liberum veto (Latin: "free veto") was a parliamentary device in the Polish-Lithuanian Commonwealth that allowed any deputy to a Sejm to force an immediate end to the current session and nullify all legislation already passed at it.

This rule evolved from a unanimity principle, and the latter from the federative character of the Polish-Lithuanian Commonwealth, which was essentially a federation of countries. Each deputy to a Sejm was elected at a local regional sejm (sejmik) and represented the entire region. He thus assumed responsibility to his sejmik for all decisions taken at the Sejm. A decision taken by a majority against the will of a minority (even if only a single sejmik) was considered a violation of the principle of political equality.

It is commonly, and erroneously, believed that a Sejm was first disrupted by means of liberum veto by a Trakai deputy, Władysław Siciński , in 1652. In reality, however, he only vetoed the continuation of the Sejm's deliberations beyond the statutory time limit. It was only in 1669, in Krakow, that a Sejm was prematurely disrupted on the strength of the liberum veto, by the Kyiv deputy, Adam Olizar .

In the first half of the 18th century, it became increasingly common for Sejm sessions to be broken up by liberum veto, as the Commonwealth's neighbors — chiefly Russia and Prussia — found this a useful tool to frustrate attempts at reforming and strengthening the Commonwealth. The latter deteriorated from a European power into a state of anarchy.

After 1764 the liberum veto practically went out of use: the principle of unanimity did not bind "confederated sejms," and so deputies formed a "confederation" (Polish: konfederacja) at the beginning of a session in order to prevent its disruption by liberum veto.

The liberum veto was abolished by the May 3rd, 1791, Constitution (adopted by a confederated sejm), which permanently established the principle of majority rule.

The achievements of that constitution, however — Europe's first written national constitution — were undone by another confederated sejm, meeting at Grodno in 1793. That Sejm, under duress from Russia and Prussia, ratified the penultimate, Second Partition of the Polish-Lithuanian Commonwealth.

See also:

• Konfederacja
• Confederated sejm
• Sejmik
• May Constitution of Poland (adopted May 3rd, 1791)