Kawakami Gensai (河上 彦斎, 25 December 1834 – 13 January 1872) was a Japanese samurai of the late Edo period.
A highly skilled swordsman, he was one of the four most notable assassins of the Bakumatsu period (the other three were Kirino Toshiaki,also known as Nakamura Hanjirō; Tanaka Shinbei, and Okada Izō). Gensai's high-speed sword discipline (in which he was the only practitioner) was called the Shiranui-ryū.
Kawakami Gensai was born in Kumamoto in 1834, to Komori Sadasuke, a retainer of the daimyo of the Kumamoto Domain. Because Gensai's older brother Hanzaemon was chosen as the family's heir, at age 11 he was given in adoption to Kawakami Genbei (河上彦兵衛), another Kumamoto retainer. He then entered the domain's school, the Jishūkan (時習館), and followed its academic and martial courses of study. Given his later prowess, it is rather curious to note that during his martial training he apparently did not win many bouts. With regards to this he is said to have commented "Kenjutsu (swordsmanship) with bamboo shinai is nothing more than play." At age 16 he was called to serve in the Kumamoto castle town as a menial in charge of cleaning (Osōji-bōzu お掃除坊主). Despite the fact that this was a low-level position, Gensai devoted himself wholeheartedly to it, using his free time to polish his martial and literary skills, as well as learn sadō (tea ceremony) and ikebana (flower arrangement). It was at this time that he met two men who would later be important in the activities of the ishin shishi: Todoroki Buhei and Miyabe Teizō. Thanks to his discussions with them, he took a serious interest in the concept of kinnō (勤王), or imperial loyalty.
In 1851, he joined the Kumamoto lord Hosokawa Narimori and went to Edo for his lord's sankin kōtai rotation. It was during his service to the lord in Edo that Commodore Perry arrived in 1853. As the shogunate subsequently entered into a series of increasingly unfair unequal treaties, Gensai left Edo in anger and returned to Kumamoto, where he entered the Gendōkan academy of the kinnō scholar Hayashi Ōen. After a thorough schooling in Ōen's kinnō philosophy, Gensai returned to Edo.
Gensai was present at the Kumamoto residence in Edo during the Ansei Purge. In the aftermath of Ii Naosuke's assassination, when a group of the escaping assassins suddenly entered the residence, it was Gensai who calmed down the subsequent uproar, calling for a doctor and having a private tea ceremony for the men. It was during this ceremony that he told the men of his admiration for them.
In 1861, Gensai married Misawa Teiko, the daughter of another Kumamoto retainer. A martial artist herself, she was highly skilled in the use of the naginata. The couple would have a son, Gentarō, who survived even after Gensai's execution, thanks to Teiko's efforts.
In 1862, he joined Kumamoto forces who were posted to security duty in Kyoto. It was at this point, he quit his job as bōzu, and soon after, left Kumamoto service altogether. In 1864, he lost his mentor Miyabe Teizō to a Shinsengumi raid at Ikedaya. Soon after, Gensai carried out his most famous and only confirmed assassination: that of Sakuma Shōzan. He killed Shōzan in one stroke, in broad daylight. While other assassinations have been attributed to him, only his murder of Shōzan can be proven.
After this, he withdrew to Chōshū and took part in the military actions of Takasugi Shinsaku's Kiheitai against the shogunate's Choshu Expeditions. However, during action in Kokura, he surrendered to Kumamoto forces, and was imprisoned until just after the Meiji Restoration.
In the aftermath of the Meiji Restoration, Gensai was released from prison. He changed his name to Takada Genbei, and served as a military official and teacher for the Kumamoto domain. However, because of his harboring of some Kiheitai stragglers under his old comrade Oraku Gentarō, he was again put in prison, and was executed in 1872.
The character Himura Kenshin from the manga Rurouni Kenshin, as well as Kawakami Bansai from the manga Gin Tama, is based on Gensai.