The Kolsås Line (Norwegian: Kolsåsbanen) is a 12.1-kilometer (7.5 mi) line of the Oslo Metro. It branches off from the Røa Line at Smestad Station and runs through western Oslo and Bærum to Kolsås Station. It serves the neighborhoods of Ullernåsen, Øraker, Jar, Bekkestua, Haslum, Gjettum and Kolsås. It is served by Line 3 of the metro at a 15-minute headway. The section from Jar to Bekkestua is built as a dual system with overhead wires, allowing Line 13 of the Oslo Tramway to continue from the Lilleaker Line to Bekkestua every ten minutes.

The Kolsås Line was built as an extension of the Lilleaker Line. It had been built to Lilleaker in 1919. The line was extended to Avløs on 1 July 1924, and the line was extended to Kolsås on 1 January 1930. The line was initially owned by Bærumsbanen, which was bought by Oslo Sporveier in 1934. A connection to the Røa Line opened on 15 June 1942, allowing the line access to the Common Tunnel. At this point the section from Sørbyhaugen to Kolsås was designated the Kolsås Line, while the section from Jar to Skøyen was retained as the Lilleaker Line. T1300 trains of the metro were introduced in 1982 and from 1995 it was linked with the eastern metro lines. The line was closed for upgrades to metro standard in 2006. It reopened in eight stages from 2008 to 2014. There are plans to extend the line to Rykkinn, Bærums Verk and Sandvika, although none of these have political priority.

The 12.1-kilometer (7.5 mi) Kolsås Line branches off from the Røa Line after Smestad Station at a site previously occupied by the now closed Sørbyhaugen Station. Sørbyhaugen is situated 5.1 kilometers (3.2 mi) from Stortinget at an elevation of 63.7 meters (209 ft). Previously there was a station at Husebybakken. The line runs parallel to Ring 3, through four stations in Oslo: Montebello, Ullernåsen, Åsjordet and Bjørnsletta. Situated at 94.6 meters (310 ft) elevation, Ullernåsen is the highest elevated station on the line. The line largely runs along Bærumsveien from slightly before Bjørnsletta.

At Øraker the Lilleaker Line meets the Kolsås Line, after which point the lines share tracks. The line crosses the municipal border across the river Lysakerelven. The now closed station of Lysakerelven, 8.4 kilometers (5.2 mi) from Stortinget, was situated on the Bærum side of the municipal border between Oslo and Bærum. At Jar there are two sets of platforms, one for the metro and one for the trams. The continues past Ringstabekk, which is only served by the metro. It replaced three disused stations, Tjernsrud, Ringstabek and Egne hjem. Situated 11.4 kilometers (7.1 mi) from Stortinget, Bekkestua is the terminal station for the tramway, and a major transit hub for central Bærum. Like Jar, it features two sets of different platforms. The shared rail section features both third-rail power supply and overhead wires. The trams have a narrower loading gauge and shorter platforms, making it necessary to provide separate platforms for them.

The line continues past Gjønnes and Haslum to Avløs, which is one of the three Oslo Metro depots. After that point, it passes through the stations of Valler to Gjettum and Hauger before reaching the terminus, Kolsås Station, which, situated 17.3 kilometers (10.7 mi) from Stortinget, is the terminus of line 3 and the station furthest away from the centre of Oslo.

The Kolsås Line is served by Line 3 of the metro. During regular hours, is operates at a 15-minute headway. It runs along the Røa Line from Smestad to Majorstuen and onwards in the Common Tunnel through the city center. East of the center the Line 3 continues along the Østensjø Line to Mortensrud. Travel time from Kolsås to Smestad is 22 minutes, and from Kolsås to Stortinget is 30 minutes. Operations are carried out by Sporveien T-banen on contract with Ruter, the public transport authority in Oslo and Akershus. The infrastructure itself is owned by Sporveien, a municipal company. Service is provided using MX3000 three- and six-car trains. In 2002 the stations along the line had an average weekday 8,568 boarding passengers. Bekkestua was the busiest with 1,126 daily boarding passengers, while Lysakerelven had the least with 235.

Jar is also served by Line 13 of the tramway. It branches off on the east side of Lysakerelven and follows the Lilleaker Line to Skøyen, where it continues as a street tram to the city center along the Skøyen Line. It operates at a ten-minute headway. Travel time to Jernbanetorget is 31 minutes from Bekkestua. In comparison, the metro uses 22 minutes from Bekkestua to Jernbanetorget. Tram operations are carried out by Sporveien Trikken using SL95 trams.

Bærum grew as a suburb of Oslo after the Drammen Line of the Norwegian State Railways opened through the southern part of the municipality in 1872. The areas around the railway stations became popular among the middle class, while the lower class started to settle further north. In particular the areas around Nadderud and Presterud received working-class populations. The construction of the settlements were often organized by trade unions. Local investors applied in 1897 to establish a tramway that would run parallel to the Drammen Line from Oslo to Sandvika—which at the time was steam-hauled, narrow gauge and single tracked and suffered from low regularity. The application was rejected by the Ministry of Labour in 1906 to avoid competition with the railway. Instead, the state started to modernize the Drammen Line by converting it to standard gauge, electrifying it and building double track.