The Seto Inland Sea, often shortened to Inland Sea, is the body of water separating Honshū, Shikoku, and Kyūshū, three of the main islands of Japan. It serves as an international 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 Sanyō Main Railroad Line, it was the main transportation link between Kansai and Kyūshū.
Yamaguchi, Hiroshima, Okayama, Hyōgo, Osaka, Kagawa, Ehime, Fukuoka, and Ōita prefectures all have coastlines on the Inland Sea; the cities of Hiroshima, Iwakuni, Takamatsu, and Matsuyama are also located on it.
The Inland Sea region is known for its moderate climate, with a stable year-round temperature and relatively low rainfall levels: The area is often called "the land of fair weather". The sea is also famous for its periodic red tides caused by dense groupings of certain phytoplankton that result in the death of large numbers of fish.
Since the 1980s, its northern and southern shores have been connected by the three routes of the Honshū-Shikoku Bridge Project, including the Great Seto Bridge, which serves both railroad and automobile traffic.