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The 32-year-old beat Russia's Valeria Koblova to win the -58kg freestyle wrestling division in Rio - as she has in every Games since Athens 2004.
Ten-time world champion Icho, whose 13-year undefeated record was ended in January, trailed with five seconds remaining but snatched victory.
Tunisia's Marwa Amri and Sakshi Malik of India won bronze medals.
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Japanese wrestler Kaori Icho has become the first woman to win individual gold medals at four consecutive Olympics.