Porsgrunn^ⓘ 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 (63 sq mi) 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. The municipality's population density is 230.5 inhabitants per square kilometre (597/sq mi) and its population has increased by 4.7% over the previous 10-year period. The conurbation of the cities of Porsgrunn and Skien is called Porsgrunn/Skien by Statistics Norway and it is considered to be the seventh-largest urban area in Norway.

The town of Porsgrunn was established as a municipality on 1 January 1838 (see formannskapsdistrikt law). The small urban town of Porsgrunn grew over time. On 1 July 1920, the growing town annexed some adjacent areas of some of the neighboring rural municipalities: an area of Gjerpen Municipality (population: 437), an area of Eidanger Municipality (population: 550), and an area of Solum Municipality (population: 1,614).

During the 1960s, there were many municipal mergers across Norway due to the work of the Schei Committee. On 1 January 1964, there was a major municipal merger where the following areas were merged to form a new Porsgrunn Municipality.

On 1 January 1968, an area of Skien Municipality (population: 3,554) was transferred to Porsgrunn Municipality (these areas had originally been part of Solum and Gjerpen municipalities).

The place is first mentioned in existing historical records 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"). The last element of the name grunn which means "ground". The name was historically spelled Porsgrund. On 1 January 1930, the spelling of the name of the municipality was changed to Porsgrunn, giving it a more "Norwegianized" spelling.

The coat of arms was originally granted on 16 January 1905 for the town of Porsgrunn. The arms were originally devised in 1905 when the city needed a new city hall. After the municipal merger on 1 January 1964, the old arms were re-adopted by the new, larger Porsgrunn Municipality. The blazon is "Gules and azure, a bend between a bog myrtle branch and an anchor argent" (Norwegian: Delt av sølv skråbjelke. Øvre felt i rødt en sølv pors-kvist, nedre felt i blått et sølv anker). This means the arms have a red (above) and blue (below) field (background) and the charge is a diagonal bend (stripe) with an anchor below it and the branch of a bog myrtle plant above it. The charge has a tincture of argent which means it is commonly colored white, but if it is made out of metal, then silver is used. The silver bend (stripe) symbolizes the small river running through the city. The bog myrtle plant is a canting symbol since the name Porsgrunn comes from the Norwegian word for the plant. The anchor on a blue background symbolizes the importance of the local harbor and sea. The arms were designed by Wilhelm Rudolph.

The Church of Norway has two parishes (sokn) within the municipality of Porsgrunn. It is part of the Skien prosti (deanery) in the Diocese of Agder og Telemark.