Minakuchi Castle (水口城 Minakuchi-jō?), also known as Hekisui Castle, is a hirashiro (castle on a plain) in Kōka, Shiga Prefecture, Japan.
Minakuchi Castle was constructed in 1634 by Tokugawa Iemitsu as a way station he could stay at during his travels between Kyoto and Edo (modern day Tokyo) on the Tōkaidō Road at the 50th stop. It was built in a similar fashion to Nijō Castle.
Katō Akitomo became lord of Minakuchi Castle in 1682 and his descendants ruled it until the Meiji Restoration, when parts were sold and dismantled. As the castle was built for the shōgun, the Katō never used the palace in the honmaru (innermost bailey), constructing instead one in the ninomaru (second bailey).
Most of Minakuchi Castle lies in ruins, though in 1991 some walls, two gates, and a yagura (turret) were reconstructed. A museum about the castle is in the yagura.
Coordinates: 34°58′14″N 136°09′52″E / 34.970446°N 136.164583°E