Koyuki began her career after winning a modeling competition in 1995 for Non-No magazine. Two years later, she became a professional model. In 1998, she landed several supporting roles in television dramas and appeared in numerous print ads and tv commercials. In 2003, Koyuki played her first starring role in a drama, Kimi wa Pet, an adaptation of Yayoi Ogawa's manga series of the same name. She plays Sumire, an intelligent, beautiful and accomplished journalist who comes in contact with a young man played by Matsumoto Jun who becomes her "pet." That same year, Koyuki made her Hollywood film debut in The Last Samurai. Koyuki plays Taka, the sister of Lord Katsumoto and wife of the red-masked Samurai Hirotaro.
In 2005, Koyuki starred in the movie Always - 3chome no Yuhi. Her performance earned her a Best Actress nomination at the Japanese Academy Awards.
On April 1st 2011 she married actor Matsuyama Kenichi who co-starred with her in Kamui Gaiden.