Kazuyo Katsuma, a 49-year-old mother of three who once featured on the Wall Street Journal's list of "The 50 Women to Watch" said she was living with Hiroko Masuhara, 40, a gay rights activist.
"I had kept the lid on my feelings of attraction to members of the same sex," Katsuma said in an interview with BuzzFeed Japan Monday.
"After I met Hiroko, the ice in my heart melted," she said, adding she hopes the revelation will "trigger a change".
Japan is largely tolerant of homosexuality but same-sex marriage is not recognised and some discrimination still exists.
Many complain they are prevented from visiting loved ones in hospitals or struggle to rent apartments because their relationship is not regarded as in line with social norms.
Katsuma said courage was still required to come out in Japan which "proves there is still prejudice and discrimination".
Her partner Masuhara, an LGBT activist, told AFP in an interview the couple was "surprised" by the extensive media coverage of the announcement.
"But I also feel that the number of allies (of LGBT people) is increasing" in Japan, Masuhara said.
Fumino Sugiyama, co-chair of the Tokyo Rainbow Pride festival said Katsuma's announcement "would have a great impact on society as she is such an influential person in business circles".
"It must have required enormous courage for her to come out," Sugiyama said.
"Sometimes I personally hear about a well-known artist or Olympian coming out but they say they are worried about the impact" on their public image, said the 36-year-old transgender activist.
In 2015, Tokyo's bustling Shibuya entertainment district started issuing symbolic "partnership certificates" to same-sex couples, in what was seen as a landmark advance for Japan's gay rights movement.
Masuhara received the first such certificate with her former partner.
Some other municipal governments have also followed suit, while corporate Japan is also showing signs of moving toward recognising same-sex couples.
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![Ana Estrada suffered from polymyositis, an incurable disease that weakens muscles [Elias Alfageme/GDA via AP]](data:image/svg+xml;charset=utf8,%3Csvg%20xmlns%3D'http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg'%20width%3D'300'%20height%3D'172'%20data-ring-placeholder%3D'1'%3E%3C%2Fsvg%3E "Ana Estrada suffered from polymyositis, an incurable disease that weakens muscles [Elias Alfageme/GDA via AP]")
