Shimazu Tadatsune (島津 忠恒; November 27, 1576 - April 7, 1638), was a tozama daimyō of Satsuma, the first Han ruler of the Shimazu clan with the establishment of the Tokugawa shogunate and the first Japanese to rule the Ryūkyū kingdom.
He is also known as Iehisa (家久), as he changed his name when he submitted to Tokugawa Ieyasu, receiving his kanji 家.
Tadatsune was the third son of Shimazu Yoshihiro. Since Yoshihiro's older brother, Shimazu Yoshihisa, had no heir son and his other older brother, Shimazu Hisakazu, had died of illness in Korea, he became his uncle's successor and later took the name Iehisa.
Like his father and uncle, he was known for bravery on the battlefield. During the second half of Hideyoshi's invasions of Korea, fighting alongside his father, he helped drive out the Ming army of over 100,000 men with only 8,000 men. As head of the Shimazu clan, he sought to remove corrupt or disloyal advisors and reform the clan's leadership.
To this end, in 1599, he killed a longtime servant and karō, Ijuin Tadamune, as well as his son, Ijuin Tadazane, when they attempted to separate from the Shimazu clan.
He became the daimyô of the Shimazu following his family's involvement in the Sekigahara campaign and went to Edo in 1602 to swear allegiance to Tokugawa Ieyasu. He received the kanji "家" from Tokugawa Ieyasu and the surname Matsudaira. In 1603 his fief was worth 605,000 koku.
In 1602 he became the head of his clan, but his father retained royal power until 1619. On April 5, 1609, Tadatsune invaded the Ryūkyū Kingdom, subjugating it and using it to trade with China.
The Ryūkyū were allowed to remain semi-independent and would not be formally annexed by Japan until after the Meiji Restoration (1868); if China learned that the Ryūkyū were controlled by the Japanese, trade would end. So Tadatsune forced this unusual status on the kingdom.
He led troops to the Osaka winter campaign and was about to participate in the Osaka summer campaign with 13,000 soldiers, but arrived after the castle had fallen.
Iehisa was confirmed in his fief in 1617.
As such, he is cited by Ueno Takafumi as the first daimyō to take part in the ritual called Sankin kōtai, imposed by the shogunate.
In 1624-1625 Iehisa again traveled to Edo, this time leaving his wife Shimazu Kameju as a political hostage; this is said to have been done at Iehisa's initiative, and marks an early example, or precedent, from which the shogunate based its later decision to compel all daimyō to leave their wives and heirs in Edo as hostages.