is a municipality in Telemark county, Norway. It is located in the traditional district of Grenland. The administrative centre of the municipality is the city of Porsgrunn. Some other notable settlements in Porsgrunn include the town of Brevik and the villages of Langangen and Heistad. The 164-square-kilometre municipality is the 315th largest by area out of the 356 municipalities in Norway. Porsgrunn is the 29th most populous municipality in Norway with a population of 37,056.