Ryūzōji Yasuie served as a bushō during the Muromachi and Sengoku periods.  He was the fourteenth head of the Ryūzōji clan of Hizen Province.

Yasuie was born as the fifth son of Ryūzōji Ieuji, the thirteenth head of the Ryūzōji clan and a kokujin, or provincial landowner, in Hizen.  His initial name was Tadatoshi or Iemitsu.

Yasuie obeyed the Kyūshū-Chiba (powerful daimyō in Hizen) and the Shōni clan, frequently engaging in battle against the Ōuchi clan.  In 1473, Yasuie revived the Jitsurin Temple on Mount Keinichi and had his son, Chōkaku, serve as the abbot.  In 1485, he issued a prohibition against forced purchases and robbery in the town of Tsu in Hizen.

At the end of the Meiō era (1492 to 1501), he transferred the headship of the clan to his second son, Ryūzōji Iekazu, and retired.  He then constructed the Mizugae mansion as his place of retirement.  Around 1505, he shaved his head and adopted the monk’s name of Oki-Nyūdō.

Yasuie died in 1510.  The Mizugae mansion was inherited by his fifth son, Ryūzōji Iekane, who renovated the premises which then became Ryūzōji Castle.  This marked the founding of the Mizugae-Ryūzōji family (an illegitimate branch of the Ryūzōji clan).