Mizuki, Earthwork fortification ruins in Dazaifu, Fukuoka Prefecture, Japan

Mizuki is an ancient fortification in Fukuoka Prefecture, Japan. The earthwork structure extends for about 1.2 kilometers and reaches a height of roughly nine meters in some sections.

Mizuki was built in 664, the year after Yamato's defeat at the Battle of Hakusukinoe (663), when fear of a Tang–Silla invasion led to defences across northern Kyushu. The Nihon Shoki records it as a large embankment in Tsukushi Province built to store water. It closed the narrowest gap between Ono Castle's plateau and the Ushikubi Plateau, and its east and west gates carried the roads from Hakata toward Dazaifu.

The name Mizuki comes from the Nihon Shoki, which records that a large embankment was built in Tsukushi Province to store water and was called Mizuki. The earthwork had an east gate for officials from the imperial court and a west gate for foreign envoys and merchants. The road through it linked the Dazaifu government center with Hakata Port until the late 8th century.

The site is about a 25-minute walk from Tofurōmae Station on the Nishitetsu Tenjin Ōmuta Line. A motorway now runs through the earthworks, and the Daifaru regional headquarters site is an empty field. A small museum on site has information in English and Japanese, and the Kyushu National Museum nearby holds a model of the fortification.

Archaeological digs brought to light pottery fragments from the 8th century bearing the site's name in old Japanese script. These finds provided proof of the place's historical identity and helped clear up centuries of uncertainty.