Porvoo, city, southern Finland, at the mouth of the Porvoo River on the Gulf of Finland, northeast of Helsinki. About one-third of the population is Swedish speaking. One of Finland’s oldest communities, it has been a trade centre since the early 14th century and received town rights in 1346. It has been the seat of a bishopric since 1723. In 1809 the Finnish Diet, in session at Porvoo, swore allegiance to Tsar Alexander I of Russia, who there granted Finland autonomy as a grand duchy. Porvoo’s development has suffered from war, fires, and its proximity to prosperous Helsinki. Best known as a cultural centre, it was the home of the national poet Johan Ludvig Runeberg and the sculptor Walter Runeberg. The medieval granite cathedral (1414–18) houses a bronze commemorating Finland’s allegiance to Alexander I. The town hall dates from 1764. Local industries include lumber mills, shipping, wood and ceramic works, and one of Finland’s largest publishing houses. Pop. (2005 est.) 46,982.