Tønsberg, historically Tunsberg, is a city in Tønsberg Municipality in Vestfold county, Norway. It is located about 102 kilometres south-southwest of the capital city of Oslo on the western coast of the Oslofjord near its mouth onto the Skagerrak. The city is the most populous metropolis in Vestfold county. Tønsberg also serves as the administrative centre for Vestfold county and the seat of the County Governor of Vestfold og Telemark.

Vestfold is a county and a current electoral district in Eastern Norway. Located on the western shore of the Oslofjord, it borders Buskerud and Telemark counties. The county administration is located in Tønsberg, Norway's oldest city, and the largest city is Sandefjord. With the exception of the city-county of Oslo, Vestfold is the smallest county in Norway by area. Vestfold was until 2019 the only county in which all municipalities had declared Bokmål to be their sole official written form of the Norwegian language.