The Instrument of Government (Swedish: 1809 års regeringsform) adopted on 6 June 1809 by the Riksdag of the Estates and King Charles XIII was one of the fundamental laws that made up the constitution of Sweden from 1809 to the end of 1974. The law came about after the Coup of 1809, when the disastrous outcome in the Finnish War led Swedish nobles and parts of the Army to revolt, forcing King Gustav IV Adolf to involuntarily abdicate and go into exile. It was thus the constitution which saw Sweden transition from an absolute monarchy of the Gustavian era (established by a previous coup in 1772) into a stable, constitutional democracy adhering to the rule of law and significant civil liberties.

Although initially coexistent with the old Riksdag of the Estates, roughly corresponding to the French Estates-General (but with four houses, the peasants being a separate estate in Sweden), it saw the advent of a United States-style bicameral legislature in 1866 which lasted until 1971. Although the 1809 Act only curtailed the powers of the king significantly, but not severely, it eventually evolved into a system largely comparable to the US Constitution, albeit with the powers of the monarchy roughly corresponding to those of the presidency, the key differences being the absence of federalism and a codified bill of rights, introduced only in the 1970s.

The Act was finally replaced by the Instrument of Government of 1974, which formally enshrined democracy and the will and equality of the people, exercised through a unicameral parliament. In this regard, the 1809 Act more closely adhered to the principles of separation of powers and dualism and remains, with 55 year between the enactment of women's suffrage in 1919 and its abolition of 1974, the most prolific constitutional arrangement of Swedish democracy.