Miyuki Hatoyama, 66, won the annual prize given to celebrities who look best in denim, along with four Japanese musicians and a Taiwanese actor.
The first lady was recognized for "wearing jeans gracefully, with a bit of a revolutionary touch," according to the Japan Jeans Association, which represents denim importers and manufacturers.
"I have long wished to win the prize, as I'm a big fan of jeans," Hatoyama said.
Japan's past first ladies made few public appearances and wore conservative dresses rarely even trousers.
Hatoyama, a strong believer in spiritualism, hasby saying she met actor Tom Cruise in a previous life and that her soul has traveled to Venus on a UFO.
The former actress said she is trying to change her husband's rather stiff and professor-like appearance.
"I've been encouraging him to wear jeans more often when he can be casual," she said.