Yukio Mishima (三島 由紀夫, Mishima Yukio?) was the pen name of Kimitake Hiraoka (平岡 公威, Hiraoka Kimitake?, January 14, 1925 – November 25, 1970), a Japanese author, poet, playwright, actor and film director, also remembered for his ritual suicide by seppuku after a failed coup d'état. Nominated three times for the Nobel Prize in Literature, Mishima was internationally famous and is considered one of the most important Japanese authors of the 20th century, whose avant-garde work displayed a blending of modern and traditional aesthetics that broke cultural boundaries, with a focus on sexuality, death, and political change.

Mishima was born in the Yotsuya district of Tokyo (now part of Shinjuku). His father was Azusa Hiraoka, a government official, and his mother, Shizue, was the daughter of a school principal in Tokyo. His paternal grandparents were Jotarō and Natsuko Hiraoka. He had a younger sister named Mitsuko, who died of typhus, and a younger brother named Chiyuki.

Mishima's early childhood was dominated by the shadow of his grandmother, Natsu, who took the boy and separated him from his immediate family for several years. Natsu was the illegitimate granddaughter of Matsudaira Yoritaka, the daimyo of Shishido in Hitachi Province, and had been raised in the household of Prince Arisugawa Taruhito; she maintained considerable aristocratic pretensions even after marrying Mishima's grandfather, a bureaucrat who had made his fortune in the newly opened colonial frontier and who rose to become Governor-General of Karafuto. She was also prone to violence and morbid outbursts, which are occasionally alluded to in Mishima's works. It is to Natsu that some biographers have traced Mishima's fascination with death. Natsu did not allow Mishima to venture into the sunlight, to engage in any kind of sport or to play with other boys; he spent much of his time alone or with female cousins and their dolls.