Harada Sanosuke (原田 左之助?, 1840 – July 6, 1868) was a Japanese warrior (samurai) who lived in the late Edo period. He was the 10th unit captain of the Shinsengumi, and died during the Boshin War.
Harada was born to a family of chūgen, or low-ranking quasi-samurai, who served the retainers of the Iyo-Matsuyama Domain (now the city of Matsuyama). He trained in the spear technique of the Hozoin-ryu style, and usually used that weapon in battle instead of a sword. During his time in Matsuyama, he was once ridiculed by a Matsuyama retainer as being a peon who was unfamiliar with how to properly commit seppuku. Harada, wishing to prove the man wrong, immediately drew his sword and attempted to commit seppuku; however, the wound was shallow, and he survived. Harada later boasted of his scar to his fellow Shinsengumi men, and the incident of his near-disembowelment is said to be the origin of the family crest he chose, which depicted a horizontal line within a circle (maru ni ichimonji (丸に一文字?)). Later leaving the Matsuyama domain's service, he went to Edo, and trained at Kondō Isami's Shieikan dōjō.
In 1863, Harada, together with Kondō and others associated with the Shieikan, joined Kiyokawa Hachirō's Roushigumi. Shortly after, Kondō and Serizawa Kamo separated from the Roushigumi, and formed the core of the group which later became the Shinsengumi.