Hoshina Masayuki was a shogunal advisor and regent to the young Tokugawa Ietsuna, who in 1651 succeeded his father Tokugawa Iemitsu, becoming shogun at the age of ten. Hoshina was the third son of Tokugawa Hidetada, half-brother to Tôfukumon-in, and uncle to the young shogun Ietsuna.
Though a son of a shogun, Masayuki was born to a concubine; as a result, Hidetada's primary wife, Oeyo, forbade him from staying within the household, and arranged to have him adopted by the Hoshina clan. His adoptive father was Hoshina Masamitsu, lord of the 30,000 koku domain of Takatô in Shinano province. Masayuki succeeded Masamitsu as lord of Takatô in 1631, but was then transferred to Yamagata han (200,000 koku) in 1636, before being transferred once again in 1644, this time to Aizu han (Wakamatsu, 230,000 koku).
In 1664, Hoshina helped resolve a domainal succession issue, when his son-in-law Uesugi Tsunakatsu, lord of Yonezawa han, died without a direct heir. Hoshina advised that the domain be split, and given to Uesugi Tsunanori. While this decision led to great financial difficulties for the domain, in the end, the Uesugi clan, continuing to feel indebted to Hoshina, and by extension to the Tokugawa clan, sided with the pro-shogunate Ôetsu Reppan Dômei in the Boshin War of 1868.
- Elizabeth Lillehoj. "A Gift for the Retired Empress." in Lillehoj (ed.). Acquisition: Art and Ownership in Edo-Period Japan. Floating World Editions, 2007. p103.
- Arai Hakuseki, Joyce Ackroyd (trans.), Told Round a Brushwood Fire, University of Tokyo Press (1979), 288n11.