Shimazu Tadatsune (島津 忠恒, November 27, 1576 – April 7, 1638) was a tozama daimyō of Satsuma, the first to hold it as a formal fief (han) under the Tokugawa shogunate, and the first Japanese to rule over the Ryūkyū Kingdom.

[1] As lord of Satsuma, he was among the most powerful lords in Japan at the time, and formally submitted to Tokugawa Ieyasu in 1602, to prove his loyalty, being rewarded as a result with the name Matsudaira Iehisa; Matsudaira being a branch family of the Tokugawa, and "Ie" of "Iehisa" being taken from "Ieyasu", this was a great honor.

Since Yoshihiro's elder brother, Shimazu Yoshihisa, did not have a son and his other elder brother, Shimazu Hisakazu, had died of illness in Korea, he was deemed successor to their uncle and he later took the name of Iehisa (家久).

[citation needed] In 1602, he became the head of Shimazu clan but his father held real power until 1619.

On April 5, 1609, Tadatsune led an expeditionary force to the Ryūkyū Kingdom, subjugating it and using it to affect trade with China.