Yamaguchi Prefecture (山口県, Yamaguchi-ken) is a prefecture of Japan located in the Chūgoku region of Honshu. Yamaguchi Prefecture has a population of 1,377,631 (1 February 2018) and has a geographic area of 6,112 km² (2,359 sq mi). Yamaguchi Prefecture borders Shimane Prefecture to the north and Hiroshima Prefecture to the northeast.

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Yamaguchi is the capital and Shimonoseki is the largest city of Yamaguchi Prefecture, with other major cities including Ube, Shūnan, and Iwakuni. Yamaguchi Prefecture is located at the western tip of Honshu with coastlines on the Sea of Japan and Seto Inland Sea, and separated from the island of Kyushu by the Kanmon Straits.