The IND Sixth Avenue Line is a rapid transit line of the B Division of the New York City Subway in the United States. It runs mainly under Sixth Avenue in Manhattan, and continues south to Brooklyn. The B, D, F, and M trains, which use the Sixth Avenue Line through Midtown Manhattan, are colored orange. The B and D trains use the express tracks, while the F, <F> and M trains use the local tracks.

The Sixth Avenue Line, constructed in stages during the 1930s, was the last trunk line built by the Independent Subway System (IND) before it was incorporated into the modern-day New York City Subway. It was more difficult to build than other subway trunk lines in New York City because construction had to proceed around, over, and under existing tunnels and elevated structures. The Sixth Avenue Line replaced the Interborough Rapid Transit Company (IRT)'s Sixth Avenue elevated, which closed in 1939. The first section of the line opened in 1936 from West Fourth Street to East Broadway with service provided by Eighth Avenue Line trains. This section was initially referred to as the Houston-Essex Street Route. The Sixth Avenue subway was completed in 1940, providing service north of West Fourth Street, connecting to the Queens Boulevard Line and the Eighth Avenue Line.

Initially, the Sixth Avenue Line carried only local service, since there were no express tracks between 34th Street and West 4th Street. In 1967 and 1968, the Chrystie Street Connection was completed, connecting the line with former BMT lines in Brooklyn via the Manhattan Bridge and with the BMT Jamaica Line over the Williamsburg Bridge. Two new stations at 57th Street and Grand Street, as well as a pair of express tracks between 34th and West 4th Streets, were built to provide the necessary capacity for the new service to Brooklyn.

There are branches on both ends of the line. On the south end, the express tracks used by the B and ​D trains diverge to Grand Street and the Manhattan Bridge. The local tracks continue through the Rutgers Street Tunnel and to York Street in Brooklyn (used by the F and <F>​ trains) or via the Chrystie Street Connection and the Williamsburg Bridge to the BMT Jamaica Line in Brooklyn (used by the M train). On the north end, north of 47th–50th Streets–Rockefeller Center, the express tracks diverge to Seventh Avenue–53rd Street and the IND Eighth Avenue Line, while a spur used by the F and ​M trains continues under Sixth Avenue to 57th Street and the 63rd Street Lines; the local tracks, used by the F and <F> train, merge with the IND Queens Boulevard Line and continue to Queens.

The following services currently use part or all of the Sixth Avenue Line, whose services' bullets are colored orange:

The majority of the Sixth Avenue Line has four tracks, two local and two express. At each end, these pairs of tracks split, giving the line two north and two south ends. One of the north ends is at 57th Street, where two tracks lead south under Sixth Avenue from the IND 63rd Street Line (used by the M train on weekdays and the F train on weekends and late nights). The other is just south of 59th Street–Columbus Circle, where a two-track line splits from the IND Eighth Avenue Line at a flying junction (with connections to the local and express tracks), immediately turns east under 53rd Street, and crosses the IND Queens Boulevard Line, which parallels it just to the north. At Seventh Avenue, the southbound track is above the northbound track (the same is true on the Queens Boulevard Line, though north is the opposite direction from the Sixth Avenue Line). These tracks are used by the B and D express trains.

The express tracks from Columbus Circle then turn south to go under Sixth Avenue, merging with the branch from 57th Street and the local tracks' split from the IND Queens Boulevard Line (used by the F local train). The branch from 57th Street merges into both the local and express track pairs; there are no direct track connections between the local and express tracks. South of this point, the Sixth Avenue Line consists of four tracks from west to east: the southbound express track, the southbound local track, the northbound express track, and the northbound local track. After passing through 47th–50th Streets–Rockefeller Center, the two southbound tracks cross each other; the tracks from Columbus Circle become the two center express tracks, and the tracks from the Queens Boulevard Line are the two outside local tracks.

South of 42nd Street–Bryant Park is a large interlocking with six crossovers and switches. The original express tracks ended just to the south at 34th Street–Herald Square and some services switched to the local tracks at the interlocking. This was done because the PATH's Uptown Hudson Tubes already existed under Sixth Avenue south of 33rd Street, and so the Sixth Avenue Line local tracks were built on each side of PATH. The section between West Fourth Street–Washington Square and 34th Street–Herald Square, the only express section of this line, was originally built as a two-track subway with the provision to expand to four tracks later. The express tracks were added in the 1960s in conjunction with the Chrystie Street Connection project. As a result, they are placed under the local tracks and PATH using the deep-bore tunneling method. At West Fourth Street–Washington Square, the express tracks return to the same level as the local tracks, and the two pairs of tracks in each direction are connected with diamond crossovers. A flying junction just to the south connects the local tracks of the Sixth and Eighth Avenue Lines. The Sixth Avenue Line then turns east under Houston Street with an express station at Broadway–Lafayette Street.