The Oslofjord Tunnel () is a subsea road tunnel which traverses the Oslofjord, connecting Hurum and Frogn in Norway. Carrying three lanes, the long tunnel reaches a depth of below mean sea level. The tunnel has a maximum gradient of seven percent. It acts as the main link connecting eastern and western Viken county, supplementing the Moss–Horten Ferry which runs further south. The tunnel is since 2018 a part of European route E134, until 2018 it was part of National Road 23.