Hiuchi Sea, Maritime region in Kagawa and Ehime Prefectures, Japan

Hiuchi Sea forms a section of the Seto Inland Sea between Kagawa and Ehime Prefectures, containing multiple islands and diverse marine environments. The waters are surrounded by coastal settlements linked together by ferry routes.

The name Hiuchi-nada appears in accounts tied to 720, when a fireball seen over Mount Akaboshi was said to calm a storm threatening a ship off Iyo. Until the modern era the shallow shores held extensive tidal flats where whales were seen; most were filled in after the war for industrial land, leaving beaches such as Kōjima and Sakurai in Imabari and Kan'onji's Kotohiki Park within Seto Inland Sea National Park.

The name Hiuchi-nada is explained by a legend: in 720, when the boat of the Iyo governor Ochi no Tamazumi nearly capsized off Doi, a fireball appeared over Mount Akaboshi and lit the sea, calming the wind. Another tradition says the spray of the strong currents was likened to sparks from a flint (hiuchi).

The sea spans roughly 60 km east to west and 40 km north to south, with an average depth of about 24 m and tides reaching some 3 m. Islands lie mostly in the north, including Ibuki and Shisaka; the southern part has none. Natural sandy beaches survive at Karako and Sakurai in Imabari, Kawarazu in Saijo and Kotohiki Park in Kanonji, all within Seto Inland Sea National Park.

The name Hiuchi-nada may come from a fireball seen over Mount Akaboshi in 720, when a storm threatened Ochi no Tamazumi's ship; other explanations cite sparks struck by the strong current. The sea is a fishing ground for sea bream, Spanish mackerel and Japanese anchovy, and finless porpoises still live here. Horseshoe crab breeding sites in Ehime are a prefectural natural monument.