Asonuma Motosato served as a bushō during the Azuchi-Momoyama period.  He was a retainer of the Mōri clan, serving as the lord of Tokonoyama Castle and head of the Asonuma clan.

Motosato was born as the lineal heir of Asonuma Motohide, a kokujin, or provincial landowner, based in the Senoarayama manor in the Aki District of Aki Province.  Asonuma Hirohide was his grandfather.

During the Keichō Campaign that began in 1596, his father, Motohide, obeyed Mōri Hidemoto and made another crossing to the Korean Peninsula (after having earlier served in the Bunroku Campaign).  After engaging in numerous battles, he participated in the urgent construction of Ulsan Castle in Ulsan, but, just prior to its completion, on 12/22 of Keichō 2 (1597), at the First Siege of Ulsan Castle, Motohide, along with other retainers of the Mōri including Reizei Motomitsu and Tsuno Ieyori, was killed in action during a sudden attack by 1,000 light infantry soldiers serving under the command of 擺寨 in the vanguard of the Chinese army.  As a result, Motosato, inherited the headship of the clan.

On 11/29 of Keichō 2 (1597), his grandfather, Hirohide, died shortly before the death of Motohide. 

On 8/3 of Keichō 6 (1601), Motosato died.  At the time, Motosato did not have an heir but his formal wife was pregnant so, on 8/26, Terumoto sent a letter to Sase Motoyoshi stating that if a son was born, then that son would inherit the Asonuma clan.  Meanwhile, if a daughter was born, a decision would be made following consultation with Mōri Hidemoto and Amano Motomasa.  The next year, a daughter was born so on 6/14 of Keichō 7 (1602), Motomasa’s son, Heishichi (later known as Asonuma Motomasa), was given a certificate for a coming-of-age ceremony and conferred the character “moto” in his name.  After marrying Motosato’s daughter, he then inherited the Asonuma clan.  In 1607, Motosato’s former wife remarried with Mōri Motoshige.