Porsgrunn is a town and municipality in the county of Telemark, Norway.
Porsgrunn was established as a municipality January 1, 1838. The town of Brevik, and the rural district of Eidanger, were merged with Porsgrunn January 1, 1964.
Porsgrunn borders the municipalities of Skien and Siljan to the north, Bamble in the west and Larvik in the east. It is part of a cluster of municipalities south in Telemark that constitute the Grenland area of Norway. The town is situated at the mouth of the river Telemarksvassdraget, also called Porsgrunnselva or The River of Porsgrunn in English, and is an important center of industry.