Tennō Seiwa was an Emperor of the early Heian period. He was the 56th sovereign of Japan.
Tennō Seiwa was born on 10 May 850 in Heian Kyo. He was the fourth son of Emperor Montoku, but in spite of the fact that he had three elder brothers, he was declared heir apparent in the year of his birth because his mother was the daughter of the powerful minister Fujiwara no Yoshifusa.
In 858 he became the fifty- sixth ruler and the first child under ten years of age to ascend the throne.
Because of his youth, all affairs of government were handled by his grandfather, Fujiwara no Yoshifusa, who in 866 was officially appointed sesslio, regent, the first person not a member of the imperial family or the Buddhist clergy to hold the title.
Soon Emperor Sciwa was unable to exercise any political power, but was completely under the control of the members of the Fujiwara family.
Yoshifusa arranged for Emperor Seiwa to marry Takaiko, the younger sister of his adopted son Mototsune, and saw to it that the son born from this union was declared emperor.
The descendants of his son Prince Sadasumi became the founders of the Genji (Minamoto family), the military leaders of the late twelfth century.