The river had a key role in the creation of Pori. Sailing in the Kokemäki river had become more and more difficult after the 14th century. The importance of Kokemäki and Ulvila began to decline when ships could no longer navigate the river. In the 16th century, the situation had become so bad that 20-years old John III (Finnish: Juhana III or Juhana-herttua) decided to establish a new harbour and market town closer to the sea. The concept and location were good, but the inhabitants were missing.
The Bourgeois of Rauma and Ulvila had been ordered to move to Helsinki, which had recently been founded, however they did not enjoy living in Helsinki, and after numerous pleas the citizens of Rauma were given permission to return to their previous homes. However, those from Ulvila were ordered to migrate to the newly founded city of Pori. On 8 March 1558 John III gave the charter of Pori, which read: "Because we have seen that it would be best to build a strong market town alongside the sea, and because we cannot find anywhere suitable for fortifying in Ulvila, we have chosen another location at Pori."
The city has burned down and been rebuilt nine times, because the houses were made of wood. First time it was destroyed by fire 1571 and last time was 1852. Only few building, like the Town hall, were saved from fire. The city plan and the shape of the present "Stone-Pori" was formed after that time. Oldest buildings are historically and culturally important (first Finnish speaking theatre) Four esplanades divide the city center in four parts and streets are wider. Main street Yrjönkatu became the first pedestrian zone in Finland 1977.
Pori National Urban Park was established in 2002. The neighboring municipality of Noormarkku was annexed with Pori on 1 January 2010