The Sognsvann Line is a rapid transit line on the Oslo Metro of Norway. It branches from the Common Tunnel at Majorstuen and runs 6.0 kilometers (3.7 mi) to Sognsvann. After Ullevål stadion, the Ring Line branches off. The Sognsvann Line serves the northwestern and northern neighborhoods of Oslo, mostly within the borough of Nordre Aker. The line is owned and maintained by Kollektivtransportproduksjon and has nine stations. The western end of line 5 serves the entire line. Line 4 and the eastern end of line 5 serve the southern part of the line up to Ullevål stadion before branching off and continuing along the Ring Line. This gives an average five-minute headway on the southern part and an average fifteen-minute headway on the northern part of the line.

A/S Holmenkolbanen was a company that owned and operated part of the Oslo Tramway and Oslo Metro in Norway from 1898 until 1975 when services were taken over by the majority owner Oslo Sporveier. Holmenkolbanen opened the Holmenkoll Line in 1898, and expanded it to become the first Nordic underground railway in 1928. The company took over operations of the Smestad Line in 1933, the Sognsvann Line in 1934. The company was merged into Oslo Sporveier in 1992.