Saito Hajime(February 18, 1844 - September 28, 1915) 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 late shogunate period.
His original position within the Shinsengumi was assistant to the vice commander. His duties included being a kenjutsu instructor. Despite prior connections to Aizu, his descendants dispute that he served as a spy. His role as an internal spy for the Shinsengumi is also questionable; one common example being that he is said to have been instructed to join Ito Kashitaro's splinter group in 1867, to spy on them. However, this is disputed by Abe Juro, who did not believe he was a spy. It is probable that he also monitored other intelligence and enemy activity. His controversial reputation comes from accounts that he executed several corrupt members of the Shinsengumi; however, rumors vary as to his role in the deaths of Takeda Kanryusai and Tani Sanjuro.
In the reorganization of the ranks in late 1864, he was first assigned as the fourth unit's captain. At Nishi Hongan-ji in April 1865 he was assigned as the third unit's captain. Saito was considered to be on the same level as the first troop captain Okita Soji and the second troop captain Nagakura Shinpachi. Together with the rest of the Shinsengumi, he became a hatamoto in 1867. After the outbreak of the Boshin War (1868-1869), Saito took part in Shinsengumi's fight during the Battle of Toba-Fushimi and the Battle of Koshu-Katsunuma, before withdrawing with the Shinsengumi's survivors to the Aizu domain.
Due to Hijikata Toshizo being incapacitated as a result of the injuries sustained at the Battle of Utsunomiya Castle, Saito became the commander of the Aizu Shinsengumi around May 26, 1868 under the name Yamaguchi Jiro (which he had used since late 1867). After the Battle of Bonari Pass, when Hijikata decided to retreat from Aizu, Saito parted with Hijikata and continued to fight with the Aizu army until the very end of the Battle of Aizu. This parting account was recorded in Kuwana retainer Taniguchi Shirobei's diary, where it was recorded as an occurrence also involving Otori Keisuke, whom Hijikata requested to take command of the Shinsengumi; thus the said confrontation was not with Hijikata. However, questions regarding this parting remain, especially considering the conflicting dates.
Saito, along with the few remaining men of the Shinsengumi who went with him, fought against the imperial army at Nyorai-do (a small temple near Aizuwakamatsu Castle), where they were severely outnumbered. Aizuwakamatsu Castle fell, Saito joined a group of former Aizu retainers who traveled southwest to the Takada Domain in Echigo Province, where they were held as prisoners of war. In the records listing the Aizu men detained in Takada, Saito is on record as Ichinose Denpachi.