Ijūin Tadaao served as a bushō during the Sengoku period. He was a retainer of the Shimazu clan.
Tadaao was born as the son of Ijūin Tadaaki, a retainer of the Shimazu.
Tadaao served Shimazu Takahisa, overseeing the kokujin, or provincial landowners, in Satsuma Province. In 1549, Tadaao facilitated a settlement between the Kabayama and Kitahara clans. Later that year, Tadaao, along with his father, Ijūin Tadaaki, deployed at the Battle of Kurokawazaki. Taking advantage of stormy weather, Tadaao led a surprise attack against Kimotsuki Kanehiro (the lord of Kajiki Castle in Ōsumi Province), prevailing in the battle and forcing his surrender.
Owing to these achievements, from 1558, Tadaao served as the head of the chief retainers of the Shimazu clan.
Tadaao was succeeded by his eldest son, Ijūin Tadamune.