Miyao Castle, Japanese castle in Hatsukaichi, Japan.

Miyao Castle sits atop a hill called Yougai-san roughly 30 meters high in the northwestern part of Itsukushima island near the main shrine. The grounds function today as a public park with two separate staircases providing access up the hillside.

Miyao Castle existed before the 1555 Battle of Miyajima: the primary source 'Tanamori Fusaaki Memorandum' first records it on 11 April 1554 (May 12, 1554), when it was occupied by Mōri forces, suggesting the Ōuchi had built it earlier. Mōri Motonari then had about 1,000 laborers work for roughly a month to strengthen it, adding stone walls to buttress the weak earthen foundation. In 1555 Sue Harukata took the fort, and Mōri retook it during the battle. Its later fate is unknown.

The hill is called Yōgai-san, and the site now holds a small shrine and an explanatory signboard. Shinto priests from the island performed ceremonies after the 1555 battle to purify the ground, since Itsukushima is sacred and the fighting was seen as a desecration.

The site is a small park atop Yougai Hill, reached by two staircases on opposite sides of the hill; one is very near the ferry docks. The hill is surrounded by residences, shops, restaurants, hotels and ryokan. There is an explanatory signboard and a small shrine at the top.

The fortress was intentionally designed as a temporary structure to serve as bait in a military trick. A substantial workforce completed the project in an exceptionally short timeframe.