Mao Kobayashi died Thursday aged 34, her husband and popular kabuki actor, Ebizo Ichikawa, said Friday at a nationally televised news conference attended by about 400 reporters.
Kobayashi shocked the country in September with her blog titled "Kokoro" - or heart -- after concealing her illness from the public for about two years.
Her blog racked up millions of registered readers on one of Japan's biggest blog platforms as she chronicled her painful battle, posting pictures of her with tubes on the face and a wig for hair loss because of anti-cancer treatment.
She was later named in the 2016 edition of the BBC's 100 Women list of inspirational and influential female figures.
"I decided to say goodbye to myself who hid behind (cancer)," Kobayashi, a mother of two, wrote in her first post.
The Mainichi Shimbun said on its front page that her blog, which attracted media attention with every update, became a social phenomenon.
The Nikkei business daily, quoting the head of a breast cancer patient support group, called her blog "courageous" because a number of cancer sufferers concealed their illness in Japan.
Japan's top government spokesman Yoshihide Suga said: "It's so regrettable (but) I'm sure that her fighting stance against the disease encouraged a lot of patients."
A tearful Ichikawa told the news conference: "She passed away just saying,'I love you.'"
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)