The town is famed for its many wooden buildings, and for its medieval cathedral which gave its name to the Porvoo Communion - an inter-church agreement between a number of Anglican and Lutheran denominationss. Its sister city in the US is Hancock, Michigan. The wooden storage buildings on the riverside are a proposed UNESCO world heritage site.
When Sweden lost the city of Viipuri to Russia in 1721, the episcopal seat was moved to Porvoo. At this time, Porvoo was the second largest city in Finland. After the conquest of Finland by Russian armies in 1808 Sweden had to cede Finland to Russia. The Diet of Porvoo in 1809 was a landmark in the History of Finland. The Tsar Alexander I confirmed the new Finnish constitution (which was essentially the Swedish constitution from 1772), and made Finland an autonomous Grand Duchy.