Nihonkai (日本海) may refer to:
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Sea of Japan
The Sea of Japan is the marginal sea between the Japanese archipelago, Sakhalin, the Korean Peninsula, and the mainland of the Russian Far East. The Japanese archipelago separates the sea from the Pacific Ocean. Like the Mediterranean Sea, it ...
, the body of water between Japan, Korea and Russia
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Nihonkai (train), an overnight sleeping car train service that ran along the Sea of Japan coastline
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