Tadamune was named his father's heir, as he was the son of one of Masamune's formal wives, while his elder half-brother, Date Hidemune, was the son of a concubine and thus less eligible to become heir. Hidemune became lord of the smaller branch domain of Uwajima han, while Tadamune went on to become lord of Sendai following Masamune's death in 1636.
Tadamune was betrothed at one time to Ichihimegimi, an infant daughter of Tokugawa Ieyasu. However, after the young princess died at the age of two, Tadamune was betrothed on 1617/12/13 instead to Furihime, a granddaughter of Ieyasu and adoptive daughter of Tokugawa Hidetada.
- Cecilia Segawa Seigle, “Tokugawa Tsunayoshi and the Formation of Edo Castle Rituals of Giving,” in Martha Chaiklin (ed.), Mediated by Gifts: Politics and Society in Japan 1350-1850, Brill (2017), 122.