Yamana Toyosada served as a shugo daimyō and sengoku daimyō in Tajima Province during the Sengoku period. He was the younger brother of Yamana Suketoyo, the head of the Tajima-Yamana clan.
In 1512, Toyosada was born as the son of Yamana Okitoyo, a daimyō and the military governor of Tajima and Bingo provinces. His formal wife was the daughter of Hosokawa Takakuni, the deputy shōgun of the Muromachi bakufu.
In 1528, after the death of his uncle, Yamana Nobutoyo, who served as the military governor of Tajima, his older brother, Suketoyo, was adopted and inherited the headship of the main branch of the Yamana clan and the role of military governor of Tajima.
Toyosada, upon orders of his older brother, Suketoyo, raided Inaba to suppress Yamana Nobumichi (the military governor of Inaba aligned with the Mōri) and made progress toward garnering control of eastern Inaba from Tottori and Iwai castles. In 1548, he killed Nobumichi and firmly established his governance of eastern Inaba. Although referenced in a historical account of Inaba, the events are not substantiated in other sources which calls their validity into question. The recent prevailing view is that Nobumichi fell from power earlier than this time and that Toyosada himself came from the capital area after the fall of Nobumichi to assume the role as the lord of Inaba. In 1552, Amago Haruhisa was formally appointed to the role of military governor of Inaba by the Ashikaga shōgun family. Based on a letter dated 4/28 of Tenbun 16 (1547) from Toyosada to Yamada Shigenao by which Toyosada granted land in the Keta District of Inaba, it has been confirmed that he entered Inaba prior to this time.
On 3/3 of Eiroku 3 (1560), Toyosada died as the age of forty-nine. After his death, Suketoyo’s son, Yamana Munetoyo, returned from the capital and served as the lord of Inaba Province.