Akiyoshidai is a Quasi-National Park in Yamaguchi Prefecture, Japan. It was founded on 1 November 1955 and has an area of 45.02 km≤.
It includes part of the Akiyoshi plateau, a 130 square kilometre area of karst topography, as well as over 400 limestone caves, including Japanís longest cave, the Akiyoshido, which has the additional status of a Special Natural Monument. The area is rated a protected landscape (category Ib) according to the IUCN. Like all Quasi-National Parks in Japan, the park is managed by the local prefectural government. The Akiyoshidai Groundwater System is a Ramsar Site and wetland of international importance.
Towards the southern end of Akiyoshidai is the Akiyoshido cave, named by Emperor Hirohito on May 30, 1926 when he was still crown prince. This spacious cave is up to 100 meters wide and has 8.79 kilometers of passages, making it the longest in Japan and one of the longest in Asia. At the present time an about one-kilometer-long section of the cave is open to the public as a sightseeing course, with a walkway and bridge system, entering at the cave's lowest point and exiting via an artificial elevator. This portion of the cave is also well decorated with a variety of large and colorful speleothems.