Isawa Castle, Historic Japanese castle ruins in Ōshū, Japan

Isawa Castle is an ancient jōsaku fortification in Ōshū, Iwate, with a grassy earthen mound at its centre. Excavations show a square enclosure 670 m on each side, ringed by a double moat and a 3.9 m earthen wall, with a 90 m government office compound inside.

Sakanoue no Tamuramaro established the fortress in 802 as a headquarters for military control of the northern territories. The compound remained operational for a century and a half before being abandoned during the Later Three Years War.

The fortress takes its name from the early Heian period administration of Japan's northern regions. Visitors today can see reconstructed gates and earthworks that show how the military compound originally appeared.

A historical park has been developed on the castle grounds, showing ancient fortification methods with reconstructed gates and earthworks. The site is accessible to the public and gives a sense of how Japanese fortresses were built in the early medieval period.

In 2005 a fragment of inked pottery bearing the characters for "mandokoro" (office) was found at the Hakusai-ji site about 400 m from the government compound. This suggests that a powerful official may have conducted administration from his own residence outside the castle walls.