Ōoku: The Inner Chambers (大奥, Ōoku) is a Japanese manga series written and illustrated by Fumi Yoshinaga. It was serialized in Hakusensha's manga magazine Melody from June 2004 to December 2020, with its chapters collected in 19 tankōbon volumes. The manga is licensed in North America by Viz Media. Ōoku: The Inner Chambers follows an alternate history of early modern Japan in which an unknown disease kills most of the male population, leading to a matriarchal society in which the Ōoku becomes a harem of men serving the now female Shogun.

It was adapted into two live-action films in 2010 and 2012 and a 10-episode Japanese television drama series in 2012. Another television drama series premiered in January 2023. An original net animation (ONA) series adaptation by Studio Deen premiered in June 2023 on Netflix.

By December 2020, the manga had over 6 million copies in circulation. Ōoku: The Inner Chambers won an Excellence Prize at the 10th Japan Media Arts Festival, a special prize at The Japanese Association of Feminist Science Fiction and Fantasy's fifth annual Sense of Gender Awards in 2005, and the Grand Prize of the 13th Tezuka Osamu Cultural Prize in 2009.

The world of Ōoku: The Inner Chambers is modeled on the Edo period in Japan, and centers on the Ōoku of Edo Castle, where the basis of social management and power is shifted from men to women as a result of the rapid decline in the male population caused by a mysterious plague.

In this work, figures who are male in history, such as the Shoguns of the Tokugawa clan for generations and those who held important positions, are replaced by women, and figures who are female by men. The story is composed of detailed descriptions based on historical facts interwoven with fiction. As for descriptions based on historical facts such as a report by an Opperhoofd: "I had an audience with Iemitsu through a blind, and I thought his voice sounded like a boy's. In the audience, only young men were seated with him. I saw many women working in the city." The rough outline in the frontispiece and columns in the magazine "Melody" reads, "Gender reversal! Parallel historical drama" or "This is a story that approximates but is not identical to the Edo period in Japan," etc., and is positioned as a so-called science fiction work (historical alteration science fiction).

During the rule of Tokugawa Iemitsu, the third Shogun of the Tokugawa shogunate, a boy was attacked by a bear in a rural village in the Kantō region, which triggered the spread of a strange disease later known as the "redface pox." No cure or treatment was ever discovered for the disease, except that it "only affects young boys" and has an "80% fatality rate." As a result, the male population plummeted to about one-quarter of the female population, and the social structure of Japan was drastically changed. Boys were raised as scarce stallions and were forced to either go to grooms, be rented out to poor families who could not get grooms at any price per night, or sell their bodies in the brothels. On the other hand, women replaced men as the main labor force, and all family businesses were handed down from woman to woman. In Edo Castle, too, the shogunship was handed over to women after the third Shogun, Iemitsu, and the Ōoku attracted endangered young men as proof of the Shogun's prestige, creating a world of men commonly referred to as "three thousand beautiful men."

The story begins with the death of the seventh Shogun, Tokugawa Ietsugu, a young girl, and the subsequent reign of the eighth Shogun, Yoshimune. She appointed her entourage Kanō Hisamichi [ja] as her intermediator [ja], and carried out various reforms of the shogunate administration. Her decisive reforms also extended to the Ōoku, where three thousand beautiful men were ridiculed as the wasteful spenders of the shogunate. One day, Yoshimune wondered why women had both female and male names. In official documents, all women were listed under male names, and even if they had husbands, they were either not mentioned, or they were purposely listed as wives under female names. She believed that this made it impossible to obtain accurate statistics for the country and harmed the shogunate administration. She met with Murase Masasuke, the head of Secretary [ja], to view the logbooks recording everything in the Ōoku. Masasuke told her that the logbooks were named the Chronicle of the Dying Day by Kasuga, the founder of the Ōoku. Yoshimune asked him what kind of man Kasuga was, but Masasuke revealed that Lady Kasuga was a woman. Yoshimune was surprised and began opening the pages of the logbooks. She thought that reading them would explain why official records used male names even though women were running the country.

The Chronicle of the Dying Day, as read by Yoshimune, dates back to the rule of the third Shogun, Iemitsu. The redface pox was spreading throughout Japan. Arikoto, a handsome monk of noble birth who had come to Edo Castle from Kyoto, was forcibly returned to secular life along with a boy monk, Gyokuei, under threat from Lady Kasuga, and was admitted to the Ōoku to become Iemitsu's page. However, it was revealed that the original Iemitsu had already died of the redface pox and that Arikoto's entry to the Ōoku was so that Chie, Iemitsu's illegitimate daughter, could give birth to an heir. The truth is that for the sake of the continuation of the Tokugawa clan, the Ōoku had been rewritten by Lady Kasuga to be populated by men.