Porsgrunn is a town and a municipality in Grenland in Vestfold and Telemark county. The municipality includes the former urban municipalities of Brevik and Porsgrunn and the rural municipality of Eidanger. Porsgrunn borders in the north to Skien and Siljan, in the east towards Larvik in Vestfold, and in the west towards Bamble. The highest point is Fjerdingen, 360 masl. The municipality is part of the area called Grenland.

The city center itself has around 13,000 inhabitants.

Porsgrunn town is located at the mouth of the Telemark watercourse and is an important industrial town. It is named after a shallow in the river that was overgrown with porcupine (Myrica gale).

Of the business activities in Porsgrunn, the process industry dominates. Previously, this business was part of Norsk Hydro, but today Yara and Ineos operate the processing plants for fertilizers and polymers, and are Porsgrunn's largest industrial employers. Yara, one of the world's largest fertilizer factories, is located here. Bilfinger Industrial Services Norway AS, Equinor, RHI Normag AS, Porsgrunds Porselænsfabrik, Isola AS, BEHA, Eramet and Norcem are other important industrial players in Porsgrunn. Many of the businesses are located in Herøya Industrial Park. For industry, all infrastructure is in place; power grids, steam, gas supply and ports. It is therefore attractive for companies to establish themselves in the municipality.

2300 students attend the University of Southeast Norway, Porsgrunn.

A double-track railway between Porsgrunn and Larvik was completed in 2018. Work is underway to connect the Vestfold line and the Sørlands line via the Grenlands line. Such a connection will make Porsgrunn one of Eastern Norway's most important hubs.

Porsgrunn is part of Gea Norvegica Geopark.