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【English Name】Seto Inland Sea

Japan's largest inland sea, embraced on three sides by western Honshu, Shikoku, and Kyushu. Its area is about 23,203 km², extending roughly 450km east to west and 15–55km north to south, with an average depth of about 38m. It is a multi-island sea containing 727 islands with a circumference of 0.1km or more, and its coast borders 11 prefectures: Yamaguchi, Hiroshima, Okayama, Hyogo, Osaka, Wakayama, Tokushima, Kagawa, Ehime, Oita, and Fukuoka.

What is noteworthy in its connection to sushi is its aspect as a rich fishing ground. The difference between high and low tide is remarkably large, locally reaching more than 2m, so the strong tidal currents stir up nutrients from the seabed and promote the growth of phytoplankton. This environment supports the habitation of more than 400–500 species of fish, and many fish important as sushi toppings—such as sea bream (tai), conger eel (anago), Spanish mackerel (sawara), and small sardines (koiwashi)—are landed here.

Its climate is called the "Seto Inland Sea climate," being mild with little precipitation, and some areas exceed 2,000 hours of annual sunshine. Since ancient times it developed as a major sea route of western Japan connecting the Kinai region and Kyushu by sea, and today it is incorporated into the Seto Inland Sea National Park.

It is also known for being home to rare creatures such as the horseshoe crab, a natural monument, and the finless porpoise (sunameri), a small cetacean, and in recent years efforts toward the restoration of seagrass beds and ecosystem conservation are being advanced.