Shimazu Tadayoshi (島津忠良; October 14, 1493-December 31, 1568) is a daimyo who ruled Satsuma Province.

He was born in a spin off clan of Shimazu clan , Mimasaka Shimazu clan(伊作島津家) but after his father Shimazu Yoshihisa died, her mother married Shimazu Unkyu of another spin off clan, Soshu clan(相州家). Tadayoshi came to represent two clan within Shimazu clan.

Shimazu Katsuhisa who presided over Shimazu clan did not have a son and he was driven out by Shimazu Sanehisa who was the head of Sasshu clan(薩州家). Sanehisa claimed to be the head of clan without properly being recognized by rest of clan. Tadayoshi had his son Shimazu Takahisa be adopted by Katsuhisa and on 1539 in the battle of Ichirai , Tadayoshi defeated Katsuhisa and had Takahisa be recognized by all member of Shimazu clan as the head.

After the succession by Takahisa, Tadayoshi retired to Kaseda of Satsuma Province. Yet he held actual power and had a trade with Ryukyu Kingdom and Ming Dynasty, massive purchases of matchlock rifles to make the clan prosperous for the eventual near unification of Kyushu by Takahisa.

Tadayoshi made a Iroha Uta poetry which sung the importance of unity. It begins with following words.

Inishie no Michi wo Kikitemo Tonaetemo Waga Okonai ni sezuba Kahinashi

いにしへの道を聞きても唱えへてもわが行いにせずばかひなし

It means "Even if you learn old ways, if you cannot use it as your own, it is meaningless". It was based on Confucianism and his educational philosophy would deeply influence his four grandsons, Shimazu Yoshihisa, Shimazu Yoshihiro, Shimazu Toshihisa , and Shimazu Iehisa . It would eventually make it way into modern philosophies in Meiji period as Satsuma han would take part in modernizing Japan.

He called himself Shimazu Nisshinsai(島津日新斎) in later years and praised his four son as "The leader Yoshihisa, the brave Yoshihiro, the planner Toshihisa, and the tactician Iehisa". He died on 1568.