The Diet of Porvoo, was the summoned legislative assembly to establish the Grand Duchy of Finland in 1809 and the heir of the powers of the Swedish Riksdag of the Estates. The session of the Diet lasted from March to July 1809.

The Grand Duchy of Finland, officially and also translated as the Grand Principality of Finland, was the predecessor state of modern Finland. It existed between 1809 and 1917 as an autonomous state ruled by the Russian Empire.