Château de Toishi, Japanese castle in Shinano Province, Japan

Château de Toishi is a fortress site in Nagano Prefecture, Japan, set on a wooded hill above the Chikuma River. The ruins show stone walls and terraces spread across several levels, offering views over the valley below.

Toishi Castle is a mountain castle (yamashiro) in the Chisagata district of Shinano, now Ueda, Nagano. Its construction date is unknown, but by the Battle of Unnodaira in 1541 it was a Murakami base, and Murakami Yoshikiyo rebuilt it extensively as a stronghold linking his home territory to Shioda and Saku. In 1550 Takeda Shingen besieged it and withdrew on 1 October, losing about a thousand men in the pursuit (the Toishi崩れ). Sanada Yukitaka took it on 26 May 1551, apparently by stratagem, and in 1553 Shingen ordered its repair. In 1600 Sanada Nobuyuki received its surrender for Tokugawa Hidetada.

The name Toishi (砥石, "whetstone") is said to have first belonged only to the southern outlying fort before it came to designate the whole complex. The site is a Nagano Prefecture-designated historic site, and the ridge position lets visitors look south over the Ueda plain and northeast toward the Sanada district.

Access is by a steep forest trail that requires sturdy footwear and can become slippery when wet. The walk takes around half an hour and passes through dense vegetation with some steep sections.

In 1550 Takeda Shingen's army abandoned its siege of Toishi after a single month, retreating on 1 October under pursuit by Murakami forces; the Katsuyama-ki records about a thousand dead, including Yokota Bitchū-no-kami. The castle fell the next year, in May 1551, when Sanada Yukitaka took it by stratagem rather than assault.