Sem is a former municipality in Vestfold county, Norway. The 102-square-kilometre (39 sq mi) municipality existed from 1838 until its dissolution in 1988. The area is now part of Tønsberg Municipality. The administrative centre was the village of Semsbyen. Other villages in Sem included Barkåker, Eik, Husvik, Husøy, Ringshaug, and Tolvsrød.

The parish of Sem was established as a municipality on 1 January 1838 (see formannskapsdistrikt law). On 1 January 1877, a small area of Sem (population: 61) was transferred into the growing city of Tønsberg. On 1 July 1915, an area of Nøtterøy Municipality (population: 12) was transferred to Sem Municipality.

During the 1960s, there were many municipal mergers across Norway due to the work of the Schei Committee. On 1 January 1965, the Stang area of northeastern Sem (population: 126) was transferred to Borre Municipality. On 1 January 1967, a minor boundary adjustment was made: an area of Stokke Municipality (population: 5) was transferred to Sem and a nearby area of Sem (population: 2) was transferred to Stokke Municipality. On 1 January 1988, all of Sem Municipality (population: 21,948) was merged with the city of Tønsberg (population: 8,896) to form a new, larger Tønsberg Municipality.

The municipality (originally the parish) is named after the old Sem farm (Old Norse: Sæmr, a compressed form of Sæheimr) since the first Sem Church was built there. The first element is sær which means "sea" or "ocean", likely referring to the local Tønsbergfjorden. The last element is heimr which means "home" or "abode".

The Church of Norway had three parishes (sokn) within the municipality of Sem. It was part of the Tønsberg domprosti (arch-deanery) in the Diocese of Tunsberg. The churches in the parish included Sem Church, Slagen Church, Vallø Church, Søndre Slagen Church, and Husøy Church.

Originally the parish of Sem was named after the historic Sem Manor (Sem hovedgård). During the Middle Ages, Sem Manor was the residence of a royal and feudal overlord at the site where Jarlsberg Manor is located today. King Harald Fairhair chose to install his son Bjorn Farmann as the master of the estate. It was here that Bjorn Farmann was killed by Eric Bloodaxe in 927. In 1673, Peder Schumacher Griffenfeld took over the property which until then had belonged to the King of Denmark. Griffenfeldt named the farm Griffenfeldgård, but three years later it was renamed Jarlsberg Manor (Jarlsberg Hovedgård). In 1682, the buildings on Jarlsberg burned and new buildings of stone were built by the new owner, the Danish-Norwegian Field Marshal Wilhelm Gustav Wedel.

While it existed, this municipality was responsible for primary education (through 10th grade), outpatient health services, senior citizen services, unemployment, social services, zoning, economic development, and municipal roads. During its existence, this municipality was governed by a municipal council of directly elected representatives. The mayor was indirectly elected by a vote of the municipal council. The municipality was under the jurisdiction of the Tønsberg District Court and the Agder Court of Appeal.

The mayors (Norwegian: ordfører) of Sem:

The municipal council (Kommunestyre) of Sem was made up of 41 representatives that were elected to four year terms. The tables below show the historical composition of the council by political party.

• List of former municipalities of Norway

• Jarlsberg Hovedgård (in Norwegian)
• Sem Church (in Norwegian)