The Port Authority Trans-Hudson (PATH) is a 13.8-mile (22.2 km) rapid transit system in the northeastern United States. It serves the northeastern New Jersey cities of Newark, Harrison, Jersey City, and Hoboken, as well as Lower and Midtown Manhattan in New York City. The PATH is operated as a wholly owned subsidiary of the Port Authority of New York and New Jersey. Trains run around the clock year-round; four routes serving 13 stations operate during the daytime on weekdays, while two routes operate during weekends, late nights, and holidays. The PATH crosses the Hudson River through cast iron tunnels that rest on a bed of silt on the river bottom. It operates as a deep-level subway in Manhattan and the Jersey City/Hoboken riverfront; from Grove Street in Jersey City to Newark, trains run in open cuts, at grade level, and on elevated track. In 2024, the system saw 62,489,400 rides, or about 221,900 per weekday in the second quarter of 2025, making it the fifth-busiest rapid transit system in the United States.

The routes of the PATH system were originally operated by the Hudson & Manhattan Railroad (H&M), built to link New Jersey's Hudson Waterfront with New York City. The system began operations in 1908 and was fully completed in 1911. Three stations have since closed; two others were relocated after a re-alignment of the western terminus. From the 1920s, the rise of automobile travel and the concurrent construction of bridges and tunnels across the river sent the H&M into a financial decline during the Great Depression, from which it never recovered, and it was forced into bankruptcy in 1954. As part of the deal that cleared the way for the construction of the original World Trade Center, the Port Authority bought the H&M out of receivership in 1962 and renamed it PATH. In the 2000s and 2010s, the system suffered longstanding interruptions from disasters that affected the New York metropolitan area, most notably the September 11 attacks and Hurricane Sandy. Both private and public stakeholders have proposed expanding PATH service in New Jersey, and an extension to Newark Liberty International Airport may be constructed in the 2020s.

Although PATH has long operated as a rapid transit system, it is legally a commuter railroad under the jurisdiction of the Federal Railroad Administration (FRA). Its right-of-way between Jersey City and Newark is located in close proximity to Conrail, NJ Transit, and Amtrak trackage, and it shares the Dock Bridge with intercity and commuter trains. All PATH train operators must therefore be licensed railroad engineers, and extra inspections are required. As of 2023^[update], PATH uses one class of rolling stock, the PA5.

The PATH system pre-dates the New York City Subway's first underground line, operated by the Interborough Rapid Transit Company. The Hudson & Manhattan Railroad (H&M) was planned in 1874, but it was not possible at that time to safely tunnel under the Hudson River. Construction began on the existing tunnels in 1890, but soon stopped when funding ran out. It resumed in 1900 under the direction of William Gibbs McAdoo, an ambitious young lawyer who had moved to New York from Chattanooga, Tennessee, and later became president of the H&M. The railroad became so closely associated with McAdoo that, in its early years, its lines were called the McAdoo Tubes or McAdoo Tunnels.

Construction started on the first tunnel, now called the Uptown Hudson Tubes, in 1873. Chief engineer Dewitt Haskin built the tunnel by using compressed air to open a space in the mud and then lining it with brick. The railroad got 1,200 feet (366 m) from Jersey City this way until a lawsuit stopped work; accidents, including a particularly serious one in 1880 that killed 20 workers, caused additional delays. The project was abandoned in 1883 due to a lack of funds. An effort by a British company, between 1888 and 1892, also failed.

When the New York and New Jersey Railroad Company resumed construction on the uptown tubes in 1902, its chief engineer, Charles M. Jacobs, used a different method. He had workers push a tunnelling shield through the mud and then place tubular cast iron plating around the tube. The northern tube of the uptown tunnel was completed this way shortly after work resumed and the southern tube was built the same way. The uptown tubes were completed in 1906.

By the end of 1904, the New York City Board of Rapid Transit Commissioners had given the company permission to build a new subway line through Midtown Manhattan to connect with the Uptown Hudson Tubes, along with 26 years of exclusive rights to the line. The Midtown Manhattan line would travel eastward under Christopher Street before turning northeastward under Sixth Avenue, then continue underneath Sixth Avenue to a terminus at 33rd Street.

In January 1905, the Hudson Companies, with $21 million in capital ($735 million in 2024), were incorporated to complete the Uptown Hudson Tubes and build the Sixth Avenue line, as well as construct a second pair of tunnels, the current Downtown Hudson Tubes. The H&M was incorporated in December 1906 to operate a passenger railroad system between New York and New Jersey via the Uptown and Downtown Tubes.