Pornainen is a small municipality of Finland. It is located in the province of Southern Finland and is part of the Uusimaa region. The neighboring municipalities are Askola in the east, Mäntsälä in the north, Porvoo in the southeast and Sipoo in the southwest. It is located 18 kilometres east of the town of Järvenpää. The municipality has a population of 4,955 and covers an area of 150.09 square kilometres of which 3.59 km2 is water.