Yui Sakano overcame team mate Atena Kodama in the women's 62-kilogram final at the Tokyo 2020 wrestling test event.
The pair had set up the showdown after impressive wins yesterday at the Olympic venue of Makuhari Messe Hall.
Sakano, the under-23 world champion in the 60kg category in 2017, proved to have the edge in the final today.
She celebrated a narrow 2-1 points win over Kodama, the 2018 world junior champion.
Ukraine's Ilona Prokopevniuk and China's Xiaojuan Luo claimed the bronze medals in the division.
Japan were denied further gold in the women's 68kg event, as Naruha Matsuyuki fell to defeat in the final.
She was beaten 2-1 by China's Xiaoqian Wang.
The podium was completed by Japan's Rin Miyaji and Ukraine's Alina Levytska.
The all-Chinese 76kg final saw China's Palinha Palinha awarded the victory by the judges, after her bout with Qian Zhou ended pointless.
Pan American Games gold medallist Justina Di Stasio of Canada won the first bronze medal in the division, with the second earned by Ukraine's Romana Vovchak.
Competition also began in the remaining three weight divisions.
China's Chun Lei and Japan's Miu Shimizu progressed to the women's 50kg final.
Another China against Japan clash will take place in the 53kg event, with Lannuan Luo set to take on the host nation's Umi Imai.
Japan are assured of success in the 57kg event, with Sena Nagamoto and Yumeka Tanabe having both reached the final.
Competition concludes tomorrow.
Venue manager Yasukazu Fukimori admitted there had been small issues, which are often raised at test events for Games.
Fukimori said there had been bugs with the printer and scoreboard at the venue on the first day of competition, but stressed these are fixable.
As with test events in other sports, the wrestling will take place under the Ready Steady Tokyo banner.