Oyama Castle (小山城, Oyama-jō) was a Japanese castle located in what is now the city of Oyama, Tochigi Prefecture, in the Kantō region of Japan. In 1991 the ruins were proclaimed a National Historic Site by the Japanese government collectively with Nakakuki Castle and Washi Castle as the "Oyama clan castle ruins". The castle is also known as Gion Castle (祇園城, Gion-jō)