East Side Access (internally abbreviated to ESA by the MTA) is a public works project in New York City that will extend the Long Island Rail Road (LIRR) from its Main Line in Queens into a new station under Grand Central Terminal on Manhattan's East Side. A project of the Metropolitan Transportation Authority (MTA), the new station and tunnels are tentatively scheduled to start service in December 2022. The project's estimated construction cost has risen nearly threefold from the planned $3.5 billion to $11.1 billion as of April 2018, making it one of the world's most expensive underground rail-construction projects.

East Side Access is based on transit plans from the 1950s, though a terminal on Manhattan's East Side was first proposed in 1963.: 17 (PDF p. 20) The planned LIRR line was included in the 1968 Program for Action of transit improvements in the New York City area. Lack of funds prevented the construction of any part of the connection other than the 63rd Street Tunnel under the East River. Plans for the LIRR connection were revived in the late 1990s. The project received federal funding in 2006, and construction began the following year. The tunnels on the Manhattan side were dug from 2007 to 2011, and the connecting tunnels on the Queens side were completed in 2012. Afterward, work began on other facilities related to the line, such as new platforms at Grand Central, ventilation and ancillary buildings, communication and utility systems, and supporting rail infrastructure in Queens. The completion of the line has been delayed several times since construction started.

The new terminal will contain eight tracks and four platforms in a two-level station 100 feet (30.5 m) below street level. It is being built in conjunction with several other expansion projects across the LIRR, including an additional track along parts of the LIRR's Main Line and a new Sunnyside station in Queens. For many riders, the new terminal is planned to remove or reduce the need for subway transfers, as many office jobs are in the Financial District and Midtown East sections of Manhattan.