Ki castle was an ancient kōgoishi type castle (also known as a Korean-style fortresses in Japan located in the city of Sōja, Okayama Prefecture, Japan. Its ruins have been protected as a National Historic Site since 1986. Portions of the castle were reconstructed in the early 2000s.

Over the course of the Yamato period, in the early centuries of the establishment of a Japanese state, a great number of Korean-style fortresses were constructed in Japan. Old fortresses dating to the 8th century and earlier can be found all over western Japan. Many of these sites have been identified with fortresses whose construction, repair, and destruction are described in detail in ancient chronicles such as Nihon Shoki and Shoku Nihongi. According to some interpretations of these texts, these fortresses were built under the guidance of, and at the orders of, various members of the Korean nobility or royalty.