Semsvannet is a lake in Asker municipality in Akershus county, southwest of Oslo, in Norway. The lake is situated close to, for the Norwegians, famous places such as the ridge Skaugumsåsen, the old cultural house Tangen and Skaugum the official residence of the heir to the throne of Norway, i.e. the crown prince or princess of Norway. The status of Semsvannet as a nature reserve was upgraded when the area was protected as a conservation area October 2, 1992. The lake and its vicinity was elected a millennium site in the year 2000.