The United States has 133 protected areas known as national monuments. The president of the United States can establish a national monument by presidential proclamation, and the United States Congress can do so by legislation. The president's authority arises from the Antiquities Act of 1906, which allows the president to proclaim "historic landmarks, historic and prehistoric structures, and other objects of historic or scientific interest" as national monuments.^[1]

Concerns about protecting mostly prehistoric Native American ruins and artifacts, collectively known as antiquities, on western federal lands prompted the legislation, which allowed the president to quickly preserve public land without waiting for legislation to pass through an unconcerned Congress. The ultimate goal was to protect all historic and prehistoric sites on U.S. federal lands,^[2] and it has resulted in designation of a wide variety of ecological, cultural and historical sites.

President Theodore Roosevelt established the first national monument, Devils Tower in Wyoming, on September 24, 1906.^[3] He established 18 national monuments, although only nine still retain that designation.^[4] Eighteen presidents have created national monuments under the Antiquities Act since the program began; only Richard Nixon, Ronald Reagan, and George H. W. Bush did not.^[5][6] Bill Clinton created 19 and expanded three others.

Jimmy Carter protected vast parts of Alaska, proclaiming 15 national monuments, 7 of which were later promoted to national parks. President Barack Obama created or expanded 34 national monuments by proclamation, the most of any president, with over half a billion acres of public land and water protected.^[7][8][5]

National monuments are located in 32 states, Washington, D.C., the Virgin Islands, American Samoa, the Minor Outlying Islands, and the Northern Mariana Islands. Arizona has the most national monuments, with 19, followed by California with 18 and New Mexico with 13. At least seventy-seven national monuments protect places of natural significance, including nineteen primarily for their geological features, eight marine sites, and eight volcanic sites (two of which are designated "National Volcanic Monuments"). At least sixty-three national monuments primarily protect historic sites, including twenty-seven associated with Native Americans, nine relating to African American history, and eleven forts. Four have been designated World Heritage Sites. With the variety of resource types there is significant variation in the size of national monuments; the median size is roughly 3,500 acres (14 km²). The five largest national monuments are all oceanic marine sites that protect waters and submerged lands where commercial fishing is prohibited.

Many former national monuments have been redesignated as national parks or another status by Congress, while others have been transferred to state control or disbanded.

Nine federal agencies in five departments manage the 133 current U.S. national monuments. Of these, 117 monuments are managed by a single agency, while 16 are co-managed by two agencies. Two of the NPS's national monuments, Grand Canyon–Parashant and Avi Kwa Ame, are not official units because they overlap with Lake Mead National Recreation Area. Management practices vary across agencies and sites according to their missions, the size or type of protected place, and legal authorization. Generally, hunting, fishing, and extraction of resources are prohibited.

Monuments and memorials that were not designated by the U.S. government, such as the USS Maine National Monument, are not listed here.

• List of national parks of the United States
• List of national memorials of the United States
• List of areas in the United States National Park System
• National Conservation Lands