The Croton Aqueduct or Old Croton Aqueduct was a large and complex water distribution system constructed for New York City between 1837 and 1842. The aqueducts, which were among the first in the United States, carried water by gravity 41 miles (66 km) from the Croton River in Westchester County to reservoirs in Manhattan. It was built because local water resources had become polluted and inadequate for the growing population of the city. Although the aqueduct was largely superseded by the New Croton Aqueduct, which was built in 1890, the Old Croton Aqueduct remained in service until 1955.

The island of Manhattan, surrounded by brackish rivers, had a limited supply of freshwater available. It dwindled as the city grew rapidly after the American Revolutionary War, and freshwater sources became polluted by effluent. Before the aqueduct was constructed, residents of New York obtained water from cisterns, wells, natural springs, and other bodies of water. Rapid population growth in the 19th century and encroachment on these areas as Manhattan moved further north of Wall Street, led to the pollution of many local fresh water sources. Below Grand Street, a small number of well-off customers of the Manhattan Company had fresh water delivered to them, but that company was actually more focused on banking—it eventually became Chase Manhattan—and only paid as much attention to its water activities as it needed to avoid losing the state charter that allowed it to bank. The poor and the rest of the city were forced to rely on well water, often made palatable by adding alcoholic spirits, prompting temperance campaigners to call for the municipal provision of water.

The unsanitary conditions caused an increase in disease. Epidemics of yellow fever ravaged the city. A polluted aquifer, overcrowded housing, the lack of sewers, public ignorance of basic sanitary conditions, and the existence of polluting industries near wells and residential areas contributed to an unprecedented mortality rate of 2.6% (1 death per 39 inhabitants) in 1830. Then in 1832 cholera first reached New York in the deadliest epidemic to that date. The need for a new supply of fresh water was crucial.

In March 1833, Major David Bates Douglass, engineering professor at West Point Military Academy, was appointed to survey and estimate the proposed route. In 1837, construction began on a massive engineering project, to divert water from sources upstate, following a route surveyed by Douglass and supervised by Douglass' successor, Chief Engineer John B. Jervis. The Croton River was dammed, aqueducts were built, tunnels dug, piping laid, and reservoirs created.

The gravity-fed aqueduct dropped 13 inches per mile, 1/4" per 100' (~0.02%). An elliptical tube, 8.5 feet (2.6m) high by 7.5 feet (2.3m) wide, of iron piping encased in brick masonry was laid, sometimes in cuts, with conical ventilating towers every mile or so, to relieve pressure and keep the water fresh. Hydraulic cement was added where the aqueduct crossed rivers. It extended from the Old Croton Dam in northern Westchester County to the Harlem River, where it continued over the High Bridge at 173rd Street and down the West Side of Manhattan and finally into a Receiving Reservoir located between 79th and 86th streets and Sixth and Seventh Avenues; the site is now the Great Lawn and Turtle Pond in Central Park. The Receiving Reservoir was a rectangular tank within fortress-like rusticated retaining walls, 1,826 feet (557 m) long and 836 feet (255 m) wide; it held up to 180 million US gallons (680,000 m³) of water. 35 million US gallons (130,000 m³) flowed into it daily from northern Westchester.

From the Receiving Reservoir, water flowed down to the Croton Distributing Reservoir, better known simply as the Croton Reservoir, a similar fortified tank located on Fifth Avenue between 40th Street and 42nd Street, where the New York Public Library Main Branch and Bryant Park are located today. This reservoir was built to resemble ancient Egyptian architecture. New Yorkers came uptown for the fine view of the city obtained from atop its walls.

The aqueduct opened to public use with great fanfare on October 14, 1842. The day-long celebration culminated in a fountain of water that spouted to a height of 50 feet (15 m) from the beautifully decorated cast iron Croton Fountain in City Hall Park.

Water started flowing through the aqueduct on June 22, 1842, taking 22 hours for gravity to take the water the 41 miles (66 km) (at a velocity of 1.86 miles per hour [2.99 km/h; 2.73 ft/s]) to reach Manhattan. Even though only 6,175 houses had been connected to the system by 1844, the Croton water had already dramatically improved both domestic hygiene and interior design. Baths and running water were being built in the private homes of wealthy New Yorkers, and public bathing facilities were constructed for the masses. The water system had another inadvertent consequence. The decline in the number of residents drawing water from the city's wells resulted in a rise in the water table, which flooded many cellars. To address this problem, the city built sewers in many residential streets. By 1852, 148 miles (238 km) of sewers had already been constructed.