"While my body was asleep, I think my soul rode on a triangular-shaped UFO and went to Venus," Miyuki Hatoyama, the wife of premier-in-waiting Yukio Hatoyama, wrote in a book published last year.
"It was a very beautiful place and it was really green."
Yukio Hatoyama is due to be voted in as premier on September 16 following his party's crushing election victory over the long-ruling Liberal Democratic Party on Sunday.
Miyuki, 66, described the extraterrestrial experience, which she said took place some 20 years ago, in a book entitled "Very Strange Things I've Encountered."
When she awoke, Japan's next first lady wrote, she told her now ex-husband that she had just been to Venus.
He advised her that it was probably just a dream.
"My current husband has a different way of thinking," she wrote. "He would surely say 'Oh, that's great'."
Yukio Hatoyama, 62, the rich grandson of a former prime minister, was once nicknamed "the alien" for his prominent eyes.
Miyuki, also known for her culinary skills, spent six years acting in the Takarazuka Revue, an all-female musical theatre group.
She met the U.S.-educated Yukio while living in America.