The Porvoo Town Hall is a neoclassical building on the old market square dating from 1764. It served as the administrative centre of the town until the 19th century and now houses part of the Porvoo Museum. The building is notable for the historical room where the Diet of Porvoo met in 1809, when Finnish estates swore allegiance to Alexander I and Finland became an autonomous Grand Duchy of Russia.
The building's white-and-yellow neoclassical exterior is typical of Finnish civic architecture of the period. The square in front of it is a pleasant gathering point in summer, often with a small craft market.