隆岩寺, A temple in Koga, Ibaraki Prefecture.

Ryuganji is a Jodo (Pure Land) school temple in the Chuo-cho (Tamachi) district of Koga, Ibaraki Prefecture. Its full name is Dairenzan Choun-in Ryuganji. The grounds contain a bell tower, the Noryu-do hall, and the adjacent Fukuju Inari Shrine. The temple also holds the memorial tower of Ogasawara Sadayoshi and an embroidered silk nehan mandala.

Ryuganji was founded in 1595 by Ogasawara Hidemasa, lord of Koga Castle, to mourn his father-in-law Matsudaira Nobuyasu, eldest son of Tokugawa Ieyasu, who was forced to take his own life on Oda Nobunaga's orders. The founding priest was Jikido Shonin. A fire destroyed the temple in 1767, and it was rebuilt afterwards. In 1793 the memorial stone of Ogasawara Sadayoshi, Hidemasa's father, was found buried nearby and moved into a stone niche here.

The adjacent Fukuju Inari Shrine, dedicated to Toyouke-hime, served as Ryuganji's guardian deity until about 1875 and was a prayer site of Ogasawara Hidemasa. The Nodoryu Hall, transferred in 1925 from Daikoin in Ota, Gunma, is also called Kosodate Nodoryu and is visited to pray for children's safe growth.

From JR Koga Station's west exit, the temple is a 10-minute walk (about 0.9 km); free rental bicycles called "Kogattsu" are available for city sightseeing. From Tobu Nikko Line's Shin-Koga Station east exit, it is a 22-minute walk (about 1.8 km).

In the grounds stands the Yamagarazuka, a grave tied to a local tale: two young samurai who fought over yamagara without their lord's order were buried alive in a deep pit, swords in hand. People say the sound of clashing blades comes from the mound at the hour of the ox on moonlit nights. The Koga Shi instead records a dispute over yamagara trapping in the Honda Tadataka era (1712-1751), when a retainer killed several men and himself, and was buried here with them.