Son of Shimotsuma Shinrai, Shimotsuma Rairyuu was a monk of Hongan Temple but said to be more devoted to politics and culture than military affairs. He had many merchant friends and joined them for tea ceremonies.

During the Ishiyama Hongan-ji War, he fought alongside his relatives Shimotsuma Yorisuke and Shimotsuma Raijun against Oda Nobunaga general Hosokawa Akimoto. In 1580 at the Ikkou Sect's surrender, he signed the peace treaty with his relatives Shimotsuma Rairen and Shimotsuma Nakataka.

A confidant of Kennyo's eldest son Kyounyo, he joined Kyounyo in a plot to retake Ishiyama Hongan Temple the following year and was rebuked by Kennyo. After Kennyo's death, Ishiyama Hongan Temple split into the Eastern Temple and Western Temple. Rairyuu followed Kyounyo to the Eastern Temple, where he became a monk magistrate. He married the daughter of Oda Nobutoki and had several children.