Mariyatsu Castle, Hill castle in Kisarazu, Japan

Mariyatsu Castle is a mountain castle in Kisarazu, Chiba Prefecture, built on a ridge of the Boso Hills at an elevation of 161 metres. Its main enclosure, called Senjōjiki, is a flat rectangular area about 50 metres east-west by 70 metres north-south, ringed by earthen ramparts, with a large embankment roughly 7 metres high on its east side. North of the main enclosure stand the secondary and tertiary enclosures, separated by deep dry moats, and the site now serves as a municipal youth nature centre and campsite.

The castle was founded in 1456 as a military outpost to control trade routes and defend territory during a period of regional conflict. Its position on the ridge made it an important point for monitoring movement across the Boso peninsula.

The castle gave its name to the Mariyatsu Takeda clan, who changed their surname from Takeda after making it their base. Under Mariyatsu Nobukiyo, who called himself "Bōsō kanrei", the clan took Koyama Castle and installed Ashikaga Yoshiaki there as the Koyama kubō. A visitor can see the castle shrine, Jōyama Jinja, below the earthen wall north of the main enclosure.

The grounds are open to walk and explore the remaining earthworks and levels. Sturdy shoes help when moving across the uneven terrain, especially after rain when the clay paths become slippery.

The castle stands on a ridge at 161 m above sea level, roughly midway between the Obitsu and Yoro river basins, about 3 km from the former and 4.4 km from the latter. Its builders chose this inland mountain site away from the Kamakura road so they could strike at either farming valley when needed.