Hosokawa Tadaoki (Japanese 細川 忠興; born November 28, 1563; died January 18, 1646) was a daimyō at the beginning of the Edo period.
Hosokawa Tadaoki was the eldest son of Hosokawa Fujitaka (細川藤高; 1534-1610). He first served Oda Nobunaga, who gave him the province of Tango in 1580.
He had married the daughter of Akechi Mitsuhide, but when Mitsuhide asked him to support his plan to eliminate Nobunaga, Tadaoki indignantly refused and hid his wife in a safe place.
During the Kyūshū campaign in 1587, his wife was baptized in Osaka under the name Gracia. In 1590, Tadaoki participated in the campaign against the Hōjō in Odawara, being assigned to capture Nirayama Castle (韮山城) in Izu Province.
After the death of Toyotomi Hideyoshi, Tadaoki, whose son had married a daughter of Maeda Toshiie, used all his influence to prevent the latter from separating from Tokugawa Ieyasu, whom he had accompanied on the campaign against Uesugi Kagekatsu.
During this time, Ishida Mitsunari ordered that all the wives and children of the Daimyō who had not separated from Ieyasu and followed him be seized and taken to Osaka Castle.
When the emissaries appeared at Hosokawa, Ogasawara, the house elder, informed his mistress that he had an order from Tadaoki to kill them rather than hand them over to the enemies of the house. Gracia submitted to her husband's wishes and died bravely in 1600 at the age of 38.
After the Battle of Sekigahara in 1600, Tadaoki was transferred from Tango to Kyūshū, where he was given Buzen Province and two districts of Bungo Province. He and his descendants resided in Kokura with a significant income of 3700 koku.
It was only from this time that Tadaoki called himself Hosokawa. During the campaign against Osaka Castle in 1614, he was supposed to observe the activities of the Shimazu clan.
But not content with that, he decisively participated in the conquest of the castle in 1615, which marked the end of the Toyotomi clan.
In 1619, Tadaoki shaved his head, called himself Sōritsu (宗立) and Sansai (三斎), respectively, and left the administration of the estate to his son Tadatoshi. He died at 81, known not only as a warrior but also as a poet.
He was also concerned with etiquette and ceremonies (有職故実, Yūsoku kojitsu), belonged to "Sen no Rikyu's seven great disciples" (千利休七哲) as a tea master, and also wrote books on the subject.