Manhattan () is the most densely populated and geographically smallest of the five boroughs of New York City. The borough is also coextensive with New York County, one of the original counties of the U.S. state of New York. Located near the southern tip of the State of New York, Manhattan is based in the Eastern Time Zone and constitutes both the geographical and demographic center of the Northeast megalopolis and the urban core of the New York metropolitan area, the largest metropolitan area in the world by urban landmass. Over 58 million people live within 250 miles of Manhattan, which serves as New York City's economic and administrative center, cultural identifier, and the city's historical birthplace. Residents of the outer boroughs of New York City often refer to Manhattan as "the city". Manhattan has been described as the cultural, financial, media, and entertainment capital of the world, and hosts the United Nations headquarters. Manhattan also serves as the headquarters of the global art market, with numerous art galleries and auction houses collectively hosting half of the world's art auctions.

Situated on one of the world's largest natural harbors, the borough consists mostly of Manhattan Island, bounded by the Hudson, East, and Harlem rivers along with several small adjacent islands, including Roosevelt, U Thant, and Randalls and Wards Islands. Manhattan additionally contains the small neighborhood of Marble Hill on the U.S. mainland, which is separated from Manhattan Island by the Harlem Ship Canal and was later connected using landfill to the Bronx. Manhattan Island is divided into three informally bounded components, each cutting across the borough's long axis: Lower, Midtown, and Upper Manhattan.

Anchored by Wall Street in the Financial District of Lower Manhattan, New York City has been called both the most economically powerful city and the leading financial and fintech center of the world, and Manhattan is home to the world's two largest stock exchanges by total market capitalization, the New York Stock Exchange and Nasdaq. Many multinational media conglomerates are based in Manhattan, and the borough has been the setting for numerous books, films, and television shows. Manhattan real estate is among the most expensive in the world, with the value of Manhattan Island, including real estate, estimated to exceed US$3 trillion in 2013; median residential property sale prices in Manhattan approximated US$1,600 per square foot ($17,000/m²) as of 2018, with Fifth Avenue in Midtown Manhattan commanding the highest retail rents in the world, at US$3,000 per square foot ($32,000/m²) per year in 2017. In 2022, the average monthly apartment rent in Manhattan climbed over US$5,000.00 for the first time.

The area of Manhattan was originally part of Lenape territory. European settlement began with the establishment of a trading post founded by colonists from the Dutch Republic in 1624 on Lower Manhattan; the post was named New Amsterdam in 1626. The territory and its surroundings came under English control in 1664 and were renamed New York after King Charles II of England granted the lands to his brother, the Duke of York. New York, based in present-day Manhattan, served as the capital of the United States from 1785 until 1790. The Statue of Liberty in New York Harbor greeted millions of immigrants as they came to America by ship in the late 19th century and is a world symbol of the United States and its ideals of liberty and peace. Manhattan became a borough during the consolidation of New York City in 1898.

New York County is the smallest county by land area in the contiguous United States, as well as the most densely populated U.S. county. Manhattan is one of the most densely populated locations in the world, with a 2020 census population of 1,694,251 living in a land area of 22.83 square miles (59.13 km²), or 72,918 residents per square mile (28,154/km²), higher than the density of any individual U.S. city. On business days, the influx of commuters increases this number to over 3.9 million, or more than 170,000 people per square mile (65,600/km²). Manhattan has the third-largest population of New York City's five boroughs, after Brooklyn and Queens, and is the smallest borough in terms of land area. If each borough were ranked as a city, Manhattan would rank as the sixth-most populous in the U.S.

Many districts and landmarks in Manhattan are well known, as New York City received a record 62.8 million tourists in 2017, and Manhattan hosts three of the world's 10 most-visited tourist attractions in 2013: Times Square, Central Park, and Grand Central Terminal. The Empire State Building has become the global standard of reference to describe the height and length of other structures. Penn Station in Midtown Manhattan is the busiest transportation hub in the Western Hemisphere. The borough hosts many prominent bridges, including the Brooklyn, Manhattan, Williamsburg, Queensboro, Triborough, and George Washington Bridges; tunnels such as the Holland and Lincoln Tunnels; skyscrapers including the Empire State Building, Chrysler Building, and One World Trade Center; and parks, such as Central Park. Chinatown incorporates the highest concentration of Chinese people in the Western Hemisphere, and Koreatown is replete with karaoke bars. The Stonewall Inn in Greenwich Village, part of the Stonewall National Monument, is considered the birthplace of the modern gay rights movement. The City of New York was founded at the southern tip of Manhattan, and the borough houses New York City Hall, the seat of the city's government. Numerous colleges and universities are located in Manhattan, including Columbia University, New York University, Cornell Tech, Weill Cornell Medical College, and Rockefeller University, which have been ranked among the top 40 in the world. The Metropolitan Museum of Art is both the largest art museum and the most visited museum in the United States. Governors Island in New York Harbor is planned to host a US$1 billion research and education center poised to make New York City the global leader in addressing the climate crisis.