Ijūin Tadaaki served as a bushō during the Sengoku period. He was a retainer of the Shimazu clan.
Tadaaki was born as the son of Ijūin Tadakimi.
Tadaaki served Shimazu Tadayoshi and Shimazu Takahisa (father and son). In 1549, at the Battle of Kurokawazaki, Tadaaki deployed along with his son, Ijūin Tadaao. Taking advantage of stormy weather, he launched a surprise attack against Kimotsuki Kanehiro (the lord of Kajiki Castle in Ōsumi Province) and forced him to surrender. In 1554, when attacking Iwatsurugi Castle, upon the recommendation of Tadaaki, the Shimazu army utilized arquebuses in battle. Owing to these achievements, Tadaaki directed political affairs for the Shimazu clan until 1556.
There is an anecdote that, in 1561, at a banquet with Kimotsuki Kanetsugu (a sengoku daimyō of Ōsumi Province), Tadaaki intentionally provoked him which led to hostilities with the Kimotsuki clan who sought at some point to take-over the Ijūin territory.
In the era of Tadaaki, the status of the Ijūin in the Shimazu clan was firmly established.