Château de Kajiyashiro, Medieval fortress in Nagasaki Prefecture, Japan

Kajiyashiro Castle is a medieval fortress in Nagasaki Prefecture, Japan, situated on a hilltop that commands views of strategic routes through the region. The structure features stone foundations and wooden reinforcements typical of feudal Japanese castles, with the natural terrain serving as part of its defensive system.

According to the Matsuura family chronicle, the castle was founded in 1069 by Minamoto no Hisa, ancestor of the Matsuura clan, when he moved from Kyoto to Matsuura district; other sources give 1095 or 1145. It was used for centuries, and in the late Sengoku period the Matsuura main line submitted to the Hirado-Matsuura. It was rebuilt by Matsuura Shigenobu for the Bunroku campaign of the 1590s, becoming a Oda-Toyotomi-style castle.

The name Kajiyashiro does not come from smithing; it is written with characters for 'kaji' and 'tani' (valley). The castle is associated with the Matsuura clan, whose ancestor Minamoto no Hisa is credited with building it. A related trace survives nearby: at Zenpuku-ji in Imabukucho, a temple bell (wani-guchi) donated in 1355 by Matsuura Tadashi, who was stationed at the castle, is a prefectural cultural property.

Visitors can reach the castle ruins by following hiking trails that lead to elevated viewpoints offering views across Nagasaki's coastal landscape. Sturdy footwear is recommended since the paths are uneven and can be slippery during or after rainfall.

At the western foot of the hill lies a residence site called Senbonmasu, where stone walls, earthen ramparts and a gate remain. Nearby Shōfuku-ji, a Shingon temple in Imabuku, keeps a bronze gong donated in 1355 by Matsuura Tadashi, then lord of Kajiyashiro.