Nishio Tadamitsu

was a daimyō in mid-Edo period Japan, who ruled Yokosuka Domain in Tōtōmi Province.

Tadamitsu was the second son of Kyōgoku Takatoyo, daimyō of Marugame Domain in Sanuki Province. As Nishio Tadanao, had no heirs, he adopted his nephew in 1729. In 1731, Tadamitsu received the courtesy title of Mondo no Shō (主水正) along with junior 5th court rank, lower grade (ju go i no ge 従五位下). Tadamitsu became head of the Nishio clan in 1760, on Tadanao's death, and was appointed Sōshaban (master of ceremonies) in the administration of the Tokugawa shogunate. He retired in 1782, and was succeeded by his son Tadayuki.

Tadamitsu died in Edo in 1789, at age 74. His grave is located at the Nishio clan temple of Ryumin-ji in modern Kakegawa, Shizuoka.