Nishio Castle dates to the Kamakura period with a fortification called Saijō-jō was built by Ashikaga Yoshiuji in around 1221.

He assigned it to one of his generals, Sakai Shigetada, who rebuilt it with moats, stone walls, several yagura, gates and a donjon.

Tanaka Yoshimasa dismantled the existing donjon and built a much larger three-story tower at the edge of the second bailey, facing the front of the castle.

The castle also had a secondary three-story donjon and ten corner yagura, which made it very well-equipped considering its size and the kokudaka of the territory.

Following the establishment of the Tokugawa shogunate, the castle became the headquarters of Nishio Domain, which changed hands many times during the Edo period between several fudai daimyō houses.