Tønsberg (pronounced [ˈtœ̂nsbær(ɡ)] ()), historically Tunsberg, is a city and municipality in Vestfold og Telemark county, eastern Norway, located around 102 kilometres (63 miles) south-southwest of Oslo on the western coast of the Oslofjord near its mouth onto the Skagerrak. The administrative centre of the municipality is the city of Tønsberg. The city is the most populous metropolis in the district of Vestfold with a population of 52,419 in 2019. The municipality has a population of 56,293 and covers an area of 329 square kilometres (127 square miles) in 2020. Tønsberg also serves as the seat for the County Governor of Vestfold og Telemark.
Tønsberg is generally regarded as the oldest city in Norway, founded by Vikings in the 9th century. Tønsberg was established as a municipality on 1 January 1838 (see formannskapsdistrikt). The rural municipality of Sem was merged into the municipality of Tønsberg on 1 January 1988. The neighboring municipality of Re was merged into Tønsberg on 1 January 2020.It is home to Tønsberg Fortress on Castle Mountain, which includes ruins from Castrum Tunsbergis, Norway's largest castle in the 13th century. An outdoor music festival is held at Tønsberg Fortress every July. Tønsberg is also home of Oseberg Mound, where the 9th-century Oseberg Ship was excavated.