A National Historic Landmark (NHL) is a building, district, object, site, or structure that is officially recognized by the United States government for its outstanding historical significance. Only some 2,500, or roughly three percent, of over 90,000 places listed on the country's National Register of Historic Places (NRHP) are recognized as National Historic Landmarks.

A National Historic Landmark District may include many contributing properties that are buildings, structures, sites or objects, and it may also include non-contributing properties. Contributing properties may or may not also be separately listed as NHLs or on the NRHP.

The origins of the first National Historic Landmark was a simple cedar post, placed by the Lewis and Clark Expedition on their 1804 outbound trek to the Pacific Ocean in commemoration of the death from natural causes of Sergeant Charles Floyd. The cedar plank was later replaced by a 100 ft (30 m) marble obelisk. The Sergeant Floyd Monument in Sioux City, Iowa, was officially designated on June 30, 1960.

Prior to 1935, efforts to preserve cultural heritage of national importance were made by piecemeal efforts of the United States Congress. In 1935, Congress passed the Historic Sites Act, which authorized the interior secretary authority to formally record and organize historic properties, and to designate properties as having "national historical significance", and gave the National Park Service authority to administer historically significant federally owned properties. Over the following decades, surveys such as the Historic American Buildings Survey amassed information about culturally and architecturally significant properties in a program known as the Historic Sites Survey.

Most of the designations made under this legislation became National Historic Sites, although the first designation, made December 20, 1935, was for a National Memorial, the Gateway Arch National Park (then known as the Jefferson National Expansion Memorial) in St. Louis, Missouri. The first National Historic Site designation was made for the Salem Maritime National Historic Site on March 17, 1938.

In 1960, the National Park Service took on the administration of the survey data gathered under this legislation, and the National Historic Landmark program began to take more formal shape. When the National Register of Historic Places was established in 1966, the National Historic Landmark program was encompassed within it, and rules and procedures for inclusion and designation were formalized. Because listings (either on the National Register, or as an NHL) often triggered local preservation laws, legislation in 1980 amended the listing procedures to require owner agreement to the designations.

On October 9, 1960, 92 places, properties, or districts were announced as eligible to be designated NHLs by U.S. Secretary of the Interior Fred A. Seaton. Agreements of owners or responsible parties were subsequently obtained, but all 92 have since been considered listed on that 1960 date.

NHLs are designated by the United States secretary of the interior because they are: