Terashima Motosada served as a bushō during the Sengoku period. Motosada was a retainer of the Jinbō clan, a daimyō family in Etchū Province. He served as lord of Ikeda Castle in the Niikawa District of Etchū. Terashima Morinori may have been an adopted son.
As a senior retainer of Jinbō Nagamoto, Motosada shared power in the family with Kojima Motoshige. In 1550, Motosada joined the side of the Nukui in an internal conflict within the Noto-Hatakeyama clan known as the Noto Tenbun Rebellion. This involved a power-struggle between senior retainers of the Noto-Hatakeyama family, namely, Yusa Tsugumitsu and Nukui Fusasada, that escalated into a battle. The Yusa called upon support from outside of Noto, turning it into an even larger conflict in which a portion of Nanao Castle in the Kashima District of Noto was burned down. Meanwhile, the Himi-Kurakawa clan (supporters of the Yusa) was eliminated, enabling the Nukui to garner control of the Noto-Hatakeyama clan.
The Terashima clan had deep ties with adherents of the Ikkō sect, while the Jinbō family had marital ties with the Shimotsuma clan who were in charge of the Hongan Temple. The Jinbō also had connections with the Takeda clan of Kai who therefore had influence within the family. However, in 1560, after the Jinbō clan was defeated by Uesugi Kenshin, Motosada came into conflict with his lord, Jinbō Nagamoto, as well as Kojima Motoshige, who both stood in opposition to the Ikkō sect. In 1566, Motosada backed Jinbō Nagazumi (the son of Jinbō Nagamoto), leading to a divisive separation of the family into two factions. From his base at Ikeda Castle, Motosada attempted to resist the Uesugi army but eventually was subdued.