The Japanese had also beaten the USA in the group stages on Friday at the Champs de Mars Arena, in the shadow of the Eiffel Tower.
Defeat means the US remains without a gold medal in the event, nicknamed “murder ball”, since 2008.
Earlier, world champions Australia claimed bronze by edging defending gold medallists Britain 50-48 in Paris.
Two-time former winners Australia lost out on the chance to fight for gold after losing to Japan in the semi-final on Sunday.
In a tight contest with the British, the Australians claimed victory to go one better than their fourth-place finish in Tokyo three years ago.
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