It is Japan's first gold medal at the London Games in the martial art it invented.
Matsumoto defeated Corina Caprioriu in the final, using footwork to gain an advantage and eventually throw the Romanian for the win.
The Japanese men have so far won two judo medals: one silver and one bronze.
The women's 57K bronze medals were won by American Marti Malloy and Automne Pavia of France.
Joliane Melancon of Laval, Que., was eliminated in the round of 16.