The National Trails System is a series of trails in the United States designated "to promote the preservation of, public access to, travel within, and enjoyment and appreciation of the open-air, outdoor areas and historic resources of the Nation". There are four types of trails: the national scenic trails, national historic trails, national recreation trails, and connecting or side trails. The national trails provide opportunities for hiking and historic education, as well as horseback riding, biking, camping, scenic driving, water sports, and other activities. The National Trails System consists of 11 national scenic trails, 21 national historic trails, over 1,300 national recreation trails, and seven connecting and side trails, as well as one national geologic trail, with a total length of more than 91,000 mi (150,000 km). The scenic and historic trails are in every state, and Virginia and Wyoming have the most running through them, with six.

In response to a call by President Lyndon B. Johnson to have a cooperative program to build public trails for "the forgotten outdoorsmen of today" in both urban and backcountry areas, the Bureau of Outdoor Recreation released a report in 1966 entitled Trails for America. The study made recommendations for a network of national scenic trails, park and forest trails, and metropolitan area trails to provide recreational opportunities, with evaluations of several possible trails, both scenic and historic. The program for long-distance natural trails was created on October 2, 1968, by the National Trails System Act, which also designated two national scenic trails, the Appalachian Trail and the Pacific Crest Trail, and requested that an additional fourteen trail routes be studied for possible inclusion. Sponsored by Senators Henry M. Jackson and Gaylord Nelson and Representative Roy A. Taylor, part of the bill's impetus was threats of development along the Appalachian Trail, which was at risk of losing its wilderness character, and the Land and Water Conservation Fund was used to acquire lands. In 1978, as a result of the study of trails that were most significant for their historic associations, national historic trails were created as a new category with four trails designated that year. Since 1968, over forty trail routes have been studied for inclusion in the system.

The scenic and historic trails are congressionally established long-distance trails, administered by the National Park Service (NPS), United States Forest Service (USFS), and/or Bureau of Land Management (BLM). These agencies may acquire lands to protect key rights of way, sites, resources and viewsheds, though the trails do not have fixed boundaries. They work in cooperation with each other, states, local governments, land trusts, and private landowners to coordinate and protect lands and structures along these trails, enabling them to be accessible to the public. These partnerships between the agency administrators and local site managers are vital for resource protection and the visitor experience. The Federal Interagency Council on the National Trails System promotes collaboration and standardization in trail development and protection. National recreation trails and connecting and side trails do not require congressional action, but are recognized by actions of the secretary of the interior or the secretary of agriculture. The national trails are supported by volunteers at private non-profit organizations that work with the federal agencies under the Partnership for the National Trails System and other trail type-specific advocacy groups.

For fiscal year 2021, the 24 trails administered by the NPS received a budget of $15.4 million.

The eleven national scenic trails were established to provide outdoor recreation opportunities and to conserve portions of the natural landscape with significant scenic, natural, cultural, or historic importance. These trails are continuous non-motorized long-distance trails that can be backpacked from end-to-end or hiked for short segments, except for Natchez Trace NST, which consists of five shorter, disconnected trail segments. The Trails for America report said, "Each National Scenic Trail should stand out in its own right as a recreation resource of superlative quality and of physical challenge." Most notably, the national scenic trail system provides access to the crest of the Appalachian Mountains in the east via the Appalachian Trail, of the Rocky Mountains in the west on the Continental Divide Trail, and of the Cascade and Sierra Nevada ranges on the Pacific Crest Trail, which make up the Triple Crown of Hiking. Other places of note include the southern wetlands and Gulf Coast on the Florida Trail, the North Woods on the North Country Trail, the variety of southwestern mountains and ecosystems on the Arizona Trail, and the remote high-mountain landscape near the Canadian border on the Pacific Northwest Trail.

They have a total length of approximately 17,800 mi (28,650 km). Due to the extent of construction of route realignments, segment alternatives, and measurement methods, some sources vary in their distances reported and values may be rounded.

Six trails are official units of the NPS, managed like its other areas, as long, linear parks. Five trails are overseen by the U.S. Forest Service.

In 2022 Arlette Laan, whose trail name was "Apple Pie", became the first woman known to have completely hiked all eleven national scenic trails.