United States National Monuments are regulated by the Federal Antiquities Act. They are managed by a variety of organizations including the National Park Service, Bureau of Land Management, and others.
As of 03/20/2020, there are 128 National Monuments in the United States1.
I would like to visit all of them, so this is a running list of the monuments I have visited and photographed.
Some of the items on this list will have links if I have written about my travels to them.
Related

Aztec Ruins National Monument

Bears Ears National Monument

Chiricahua National Monument

Canyon de Chelly
National Monument

Carrizo Plain National Monument

Cedar Breaks National Monument

Gila Cliff Dwellings National Monument

Fort Union National Monument

Gold Butte
National Monument

Hovenweep National Monument
Mojave Trails National Monument
Natural Bridges National Monument
Organ Pipe Cactus National Monument
Pipe Spring National Monument
San Juan Islands National Monument
Santa Rosa and San Jacinto Mountains National Monument

Tonto National Monument

Walnut Canyon National Monument

Bandelier National Monument

Cabrillo National Monument

Capulin Volcano National Monument

Canyons of the Ancients
National Monument

Casa Grande Ruins National Monument

Colorado National Monument

Devils Postpile National Monument

Giant Sequoia National Monument

Grand Staircase-Escalante
National Monument

Lava Beds National Monument
Montezuma Castle National Monument
Navajo National Monument
Petroglyph National Monument
San Gabriel Mountains National Monument
Sand To Snow National Monument
Sunset Crater Volcano National Monument
Tuzigoot National Monument

Vermilion Cliffs National Monument

Wupatki National Monument