Amakusa Sea, Marine area between Nagasaki, Kumamoto, and Kagoshima Prefectures, Japan.

The Amakusa Sea is a body of water in southwestern Japan, located between Nagasaki, Kumamoto, and Kagoshima Prefectures, forming part of the East China Sea. It is dotted with small islands and divided by a series of narrow straits, shallow bays, and rocky coastlines that connect the island groups of the area.

The sea is named in the Edo period: the Y阳明 scholar Rai San'yō wrote a poem about it, and the modern novelist Hayashi Fumiko later used "Amakusa-nada" as the title of a novel. The waters also appear in the Japanese nautical charts issued by the Japan Coast Guard, which set a baseline between Kōjima and Ue-Kamijima, making the sea northeast of that line Japan's internal water.

The sea appears in Japanese literature: Fumiko Hayashi's novel "Amakusa Nada" and poems by the Edo-period scholar Rai San'yo both take it as their subject. Along the coast, fishing is still practised, with sardines, mackerel, red sea bream, dolphinfish, spiny lobster, octopus and squid among the catch.

The sea lies within Japan's internal waters north-east of a baseline drawn between Kōjima, south-east of Fukue Island, and Ueshima in the Koshikijima Islands; up to 12 nautical miles south-west of this line is Japanese territorial sea. The coasts of Shimoshima, the Shimabara and Nagasaki Peninsulas, and the Goto Islands enclose the water. Strong currents make the sea often rough.

Wild bottlenose dolphins live in these waters year-round and can be spotted on boat tours that operate out of the nearby ports. The local dolphin population is large enough that sightings are considered reliable rather than rare, which makes this one of the more dependable spots in Japan for watching them in the open sea.