The United States Department of the Interior (DOI) is an executive department of the U.S. federal government responsible for the management and conservation of most federal lands and natural resources. It also administers programs relating to Native Americans, Alaska Natives, Native Hawaiians, and four of the five inhabited insular areas of the United States, as well as programs related to historic preservation. About 75% of federal public land is managed by the department, with most of the remainder managed by the Department of Agriculture's Forest Service. The department was created on March 3, 1849. It is headquartered at the Main Interior Building, located at 1849 C Street NW in Washington, D.C.

The department is headed by the secretary of the interior, who reports directly to the president of the United States and is a member of the president's Cabinet. The current interior secretary is Doug Burgum, who was sworn in on February 1, 2025.

As of mid-2004, the department managed 507 million acres (2,050,000 km²) of surface land, or about one-fifth of the land in the United States. It manages 476 dams and 348 reservoirs through the Bureau of Reclamation, 433 national parks, monuments, historical sites, etc. through the National Park Service, and 544 national wildlife refuges through the Fish and Wildlife Service. The largest land management agency is the Bureau of Land Management, managing about one-eighth of the land in the United States.

Despite its name, the Department of the Interior has a different role from that of the interior ministries of other nations, which are usually responsible for police matters and internal security. In the United States, national security and immigration functions are performed by the Department of Homeland Security primarily and the Department of Justice secondarily. The Department of the Interior has often been humorously called "the Department of Everything Else" because of its broad range of responsibilities.

A department for domestic concern was first considered by the 1st United States Congress in 1789, but those duties were placed in the Department of State. The idea of a separate domestic department continued to percolate for a half-century and was supported by presidents from James Madison to James Polk. The 1846–48 Mexican–American War gave the proposal new steam as the responsibilities of the federal government grew. Polk's secretary of the treasury, Robert J. Walker, became a vocal champion of creating the new department.

In 1849, Walker stated in his annual report that several federal offices were placed in departments with which they had little to do. He noted that the United States General Land Office had little to do with the Treasury and also highlighted the Indian Affairs office, part of the Department of War, and the Patent Office, part of the Department of State. Walker argued that these and other bureaus should be brought together in a new Department of the Interior.^{[citation needed]} A bill authorizing its creation of the department passed the House of Representatives on February 15, 1849, and spent just over two weeks in the Senate. The department was established on March 3, 1849 (9 Stat. 395), the eve of President Zachary Taylor's inauguration, when the Senate voted 31 to 25 to create the department. Its passage was delayed by Democrats in Congress who were reluctant to create more patronage posts for the incoming Whig administration to fill. The first secretary of the interior was Thomas Ewing.

Several of the domestic concerns the department originally dealt with were gradually transferred to other departments. For example, the Department of Interior was responsible for water pollution control prior to the creation of the Environmental Protection Agency. Other agencies became separate departments, such as the Bureau of Agriculture, which later became the Department of Agriculture. However, land and natural resource management, American Indian affairs, wildlife conservation, and territorial affairs remain the responsibilities of the Department of the Interior.

Secretary of the Interior Albert B. Fall was implicated in the Teapot Dome scandal of 1921. He was convicted of bribery in 1929, and served one year in prison, for his part in the controversy. A major factor in the scandal was a transfer of certain oil leases from the jurisdiction of the Department of the Navy to that of the Department of the Interior, at Fall's behest.