From the USGS:
The Protected Areas Database of the United States (PAD-US) is the official inventory of public parks and other protected areas in all U.S. states and territories.
The growing database contains more than three billion public land and marine acres managed by nearly 15,100 agencies and nongovernmental organizations, covering 200,000 separate parks and protected areas. PAD-US is a product of Core Science Analytics, Synthesis, and Library (CSAS&L) in the USGS Core Science Systems Mission Area.
What can you do with PAD-US?
Explore the different types of public lands in the United States. National, state, regional, and local organizations all manage protected lands. From the PAD-US map you can identify protected areas managed by various federal agencies such as the Bureau of Land Management, the U.S. Forest Service, and the National Park Service. Or you can view information for recreational areas such as State Parks.
Direct to Protected Areas Database of the United States (PAD-US)
See Also: PAD-US—National Inventory of Protected Areas Fact Sheet
2 pages; PDF (Revised August 4, 2017)
See Also: Completing America’s Inventory of Public Parks and Protected Areas (via USGS)
60 pages; PDF.