Tsuda Sanzō (津田 三蔵, Tsuda Sanzō, February 15, 1855 – September 30, 1891) was a Japanese policeman who in 1891 attempted to assassinate the Tsesarevich Nicholas Alexandrovich of Russia (later Emperor Nicholas II), in what became known as the Ōtsu incident.

Born into a samurai family in the village of Kanayama, his ancestors were doctors to the daimyōs of Iga.

At the trial, Tsuda indicated that he had attempted to assassinate Nicholas because he suspected that he was a Russian spy.

Tsuda was sentenced to life imprisonment on May 25, 1891, which he had to serve in Hokkaido, often called "the Japanese Siberia".

[1] In his native village after the incident, it was forbidden to give newborn children the name Tsuda, and his family became outcasts.