Porsgrunn is a town and municipality in Telemark county, Norway. It is part of the traditional region of Grenland. The administrative centre of the municipality is the town of Porsgrunn. The municipality of Porsgrunn was established on 1 January 1838 (see formannskapsdistrikt). The town of Brevik and the rural district of Eidanger were merged into the municipality of Porsgrunn on 1 January 1964.
The place is first mentioned in 1576 ("Porsgrund") by the writer Peder Claussøn Friis in his work Concerning the Kingdom of Norway (see the article: Norwegian literature). He writes: "Two and a half miles from the sea, the Skien river flows into the fjord, and that place is called Porsgrund." The name was probably given during medieval times to the then swampy area by the nuns of Gimsøy Abbey, who went here to collect the shrub pors ("Bog Myrtle"), and the Norwegian word grunn meaning "ground". Prior to 1931, the name was spelled "Porsgrund".