Yellowstone National Park

The oldest national park in the United States is Yellowstone National Park, located mostly in northwestern Wyoming and partially in Montana and Idaho. Established by legislation passed by the U.S. Congress and signed by President Ulysses S. Grant on March 1, 1872, it is widely considered the first national park in the world
(Note: some have indicated there is evidence that Bogd Khan Mountain National Park in Mongolia was first dating back to 1778).
The United States has 63 national parks, all operated by the National Park Service, which is an agency of the U.S. Department of the Interior, and California has the largest number (9) of any state in the United States.