Porsgrunn is a city and municipality in Telemark county, Norway. It is part of the traditional region of Grenland. The administrative centre of the municipality is the city of Porsgrunn. The municipality of Porsgrunn was established on 1 January 1838. The town of Brevik and the rural district of Eidanger were merged into the municipality of Porsgrunn on 1 January 1964.
The conurbation of Porsgrunn and Skien is considered by Statistics Norway to be the seventh-largest city in Norway.