The Flatiron Building, originally the Fuller Building, is a 22-story, 285-foot-tall (86.9 m) steel-framed triangular building at 175 Fifth Avenue in the Flatiron District neighborhood of Manhattan in New York City. Designed by Daniel Burnham and Frederick P. Dinkelberg, and sometimes called, in its early days, "Burnham's Folly", it was opened in 1902. The building sits on a triangular block formed by Fifth Avenue, Broadway, and East 22nd Street—where the building's 87-foot (27 m) back end is located—with East 23rd Street grazing the triangle's northern (uptown) peak. The name "Flatiron" derives from its triangular shape, which recalls that of a cast-iron clothes iron.

The Flatiron Building was developed as the headquarters of construction firm Fuller Company, which acquired the site from the Newhouse family in May 1901. Construction proceeded rapidly, and the building opened on October 1, 1902. Originally 20 floors, a "cowcatcher" retail space (a low attached building so called for its resemblance to the device on rail locomotives) and penthouse were added shortly after the building's opening. The Fuller Company sold the building in 1925 to an investment syndicate. The Equitable Life Assurance Society took over the building after a foreclosure auction in 1933 and sold it to another syndicate in 1945. Helmsley-Spear managed the building for much of the late 20th century, renovating it several times. The Newmark Group started managing the building in 1997. Ownership was divided among several companies, which started renovating the building again in 2019. Jacob Garlick agreed to acquire the Flatiron Building at an auction in early 2023, but failed to pay the required deposit, and three of the four existing ownership groups took over the building. In October 2023, the building's owners announced that it would be converted to residential condominiums; the project is planned to be complete by 2026.

The Flatiron Building's facade is divided vertically into three sections, similarly to the components of a classical column. The three-story base is clad with limestone, while the upper stories are clad with glazed terracotta. The building's steel frame, designed by structural engineering firm Purdy and Henderson, was intended to withstand four times the maximum wind force of the area. Called "one of the world's most iconic skyscrapers and a quintessential symbol of New York City", the building anchors the south (downtown) end of Madison Square and the north (uptown) end of the Ladies' Mile Historic District. The neighborhood around it is called the Flatiron District after its signature, iconic building. The building was designated a New York City landmark in 1966, was added to the National Register of Historic Places in 1979, and was designated a National Historic Landmark in 1989. A survey in 2023 found that the Flatiron Building was the fourth-most-loved building in the United States.