Other
- Yamaguchi-gumi, a yakuza group
- Yamaguchi Bank, a major regional bank in Japan, primarily in the Yamaguchi prefecture
- Yamaguchi Broadcasting (later named KRY), a Japanese television and radio broadcasting company serving the city of Yamaguchi and Yamaguchi Prefecture
- Yamaguchi Bicycles, a custom bike framebuilder based in Rifle, Colorado
- Sweden v. Yamaguchi, a highly complex decision in international family law which touches on questions in law still unresolved over fifty years later
- 15841 Yamaguchi, an asteroid named for the Japanese prefecture
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