Niiro Ryoan served as a bushō during the Sengoku and Azuchi-Momoyama periods.  He was a retainer of the Shimazu clan.  Ryoan served as lord of the manor of Takaharu in Hyūga Province, Kurino in Ōsumi Province, and Ichiki in Satsuma Province.

In 1553, Ryoan was born as the third son of Niiro Yasuhisa of an illegitimate branch of the Niiro clan.  In his youth, Ryoan entered the priesthood and made a pilgrimage across numerous provinces as a follower of the Jishū branch of the Jōdo sect of Buddhism.  Later, he became the abbot at the Shōgon Temple in Yatsushiro in Higo Province and adopted the name of Ryoan.

In 1587, his older brother, Niiro Hisaatsu, came multiple times upon orders of Shimazu Yoshihisa to convince him to return to secular life whereupon he became a chief retainer of Shimazu Yoshihiro.  For the Bunroku-Keichō Campaign on the Korean Peninsula and the Battle of Sekigahara, he followed Yoshihiro on deployment.  During the Battle of Sekigahara, he headed toward the territory of Torii Mototada who was holed-up in Fushimi Castle and requested to enter the castle but was denied.

While withdrawing from Sekigahara, he separated from the party of Yoshihiro and went into hiding in Kurama.  He was then located and apprehended by Yamaguchi Naotomo, the investigator of fleeing soldiers from the fallen enemy, serving Tokugawa Ieyasu.  During interrogation by Ieyasu, Ryoan was ordered to urge Shimazu Yoshihisa and Shimazu Tadatsune to travel to Kyōto and released.

After the Battle of Sekigahara, Ryoan made concerted efforts in negotiations with Ieyasu to obtain recognition of the rights of the Shimazu to their territory.  Following a reconciliation, Ryoan abided by Tadatsune to travel to Kyōto and, in 1602, died of illness in Ōsaka.  This occurred soon after notification that Ieyasu would recognize the rights of the Shimazu to their territory.

In his childhood, Ryoan received baptismal rites from Francisco de Xavier, a Jesuit missionary from Spain, during de Xavier’s visit to Satsuma.