Setonaikai National Park is a Japanese national park, comprising areas of Japan's Seto Inland Sea, and of ten bordering prefectures. Designated a national park in 1934, it has since been expanded several times. It contains about 3,000 islands, known as the Setouchi Islands, including the well-known Itsukushima. As the park encompasses many non-contiguous areas, and covers a tiny proportion of the Inland Sea's total extent, control and protection is problematic; much of the wider area is heavily industrialized.

The Seto Inland Sea , sometimes shortened to the Inland Sea, is the body of water separating Honshu, Shikoku, and Kyushu, three of the four main islands of Japan. It serves as a waterway connecting the Pacific Ocean to the Sea of Japan. It connects to Osaka Bay and provides a sea transport link to industrial centers in the Kansai region, including Osaka and Kobe. Before the construction of the San'yō Main Line, it was the main transportation link between Kansai and Kyūshū.