Saitō Hajime was a Japanese samurai of the late Edo period, who most famously served as the captain of the third unit of the Shinsengumi. He was one of the few core members who survived the numerous wars of the Bakumatsu period. He was later known as Fujita Gorō and worked as a police officer in Tokyo during the Meiji Restoration where he worked mostly undercover for them along with the Japanese government.

The Shinsengumi was a small, elite group of swordsmen that was organized by commoners and low rank samurai, commissioned by the bakufu during Japan's Bakumatsu period in 1863. It was active until 1869. It was founded to protect the shogunate representatives in Kyoto at a time when a controversial imperial edict to exclude foreign trade from Japan had been made and the Chōshū clan had been forced from the imperial court. They gained considerable fame in the Ikedaya incident and the August 18 coup events, among others. The men were drawn from the sword schools of Edo.