Shimazu Tadatsune (島津忠恒; November 27, 1576-April 7, 1638) was the third son of Shimazu Yoshihiro and the first ruler of Satsuma han.

Even though he was only the third son, his elder brother Shimazu Yoshihisa did not have a son and his elder brother Shimazu Hisakazu died of illness in Korea, he was deemed the succesor of the Yoshihisa and he later took the name of Iehisa (家久).

Like his father and uncle, he was known for bravery on the battle field, and during the latter half of the Seven-Year War, fighting beside his father, he drove off the Ming army existing of over 100,000 men with only 8000 men. In 1599, he killed a long-time retainer and karo, Ijuin Tadamune as well as his son Ijuin Tadazane when they tried to part with the Shimazu clan.

In 1602, he became the head of his clan but his father held real power until 1619. In 1609, he led an expeditionary force to the Ryukyu Kingdom to subjugate it and to start a secret trade route with the Ming Dynasty. He ordered a town plan as well as other numerous measures to transform the warring clan into a peaceful and prosperous state. He also sent his wife and children to Edo as sign of subjugation to the Tokugawa shogunate. He died in 1638.