The Interborough Rapid Transit Company (IRT) was the operator of the original underground New York City Subway line that opened in 1904, as well as earlier elevated railways and additional rapid transit lines in the City of New York. It is now the Subway's A Division, running the numbered services and the 42nd Street Shuttle.

The first IRT subway ran between City Hall and Broadway at 145th Street , opening on October 27, 1904. It opened following more than twenty years of public debate on the merits of subways versus the existing elevated rail system and on various proposed routes.

It acquired the pre-existing Manhattan Elevated by lease, gaining a monopoly on rapid transit in Manhattan. The Manhattan el was the operator of four elevated railways in Manhattan with an extension into The Bronx. The IRT coordinated some services between what became its subway and elevated divisions, but all the lines of the former Manhattan el have since been dismantled.

The IRT today operates as a subway in Manhattan, except for a short stretch across Harlem at 125th Street and in northernmost Manhattan. Its many lines in the Bronx are predominantly elevated, with some subway, and some railroad-style right-of-way inherited from the defunct New York, Westchester and Boston Railway , which now comprises the Dyre Avenue Line. Its two Brooklyn lines are in subway with a single elevated extension that reaches into Queens, and the Flushing Line, its sole major Queens line is entirely elevated except for a short portion approaching its East River tunnel and its terminal in Flushing. The Flushing Line has had no physical connection to the rest of the IRT since 1942. It is connected to the BMT at Queensborough Plaza and major servicing in done at the BMT's Coney Island Yard.

The IRT was purchased by the City in June 1940. The former IRT lines (the numbered lines in the current subway system) are now the IRT Division of the New York City Transit Authority, also known as the A Division.