International Opera Competition, the first Japanese to take top honors in the contest, according to organizers.
At the final elimination Sunday in Hamamatsu, Shizuoka Prefecture, Mitsuoka also won the audience prize and the special prize commemorating the late Tamaki Miura, a world-renowned diva and native of Shizuoka Prefecture.
A member of the famed Fujiwara Opera troupe and graduate of Showa Academy of Music, Mitsuoka performs in Japan and abroad. She lives in Zurich.
The second and third prizes went to South Korean sopranos Lee Hye Jung, 25, and Kim Nam Young, 26.
Lee and Kim study in New York and Germany, respectively. The prizewinners held a special concert in Tokyo on Thursday evening.
Sixty contestants from nine countries were chosen from more than 300 hopefuls from 33 countries who auditioned for the competition, which began Nov. 1.