Asonuma Motomasa served as a bushi during the Azuchi-Momoyama and early Edo periods.  He was a retainer of the Mōri clan and the Chōshū domain.

Motomasa was born as the son of Amano Motomasa who was, in turn, the seventh son of Mōri Motonari.

On 8/3 of Keichō 6 (1601), Asonuma Motosato, a retainer of the Mōri, died.  At the time, Motosato died without an heir and his formal wife was pregnant so, on 8/26, Mōri Terumoto sent a letter to Sase Motoyoshi stating that if a son was born, then he would have that son inherit the Asonuma clan but if a daughter was born, then a successor would be determined on the basis of consultations with Mōri Hidemoto and Amano Motomasa.  The next year, after a daughter was born, Terumoto consulted with Hidemoto and Motomasa whereby Motomasa’s son, Heishichi (later known as Motomasa) wed Motosato’s daughter and Motomasa inherited the Asonuma clan as an adopted son-in-law with a stipend of 2,500 koku.

On 6/14 of Keichō 7 (1602), during his coming-of-age ceremony, he received a certificate from Terumoto conferring the character “moto” whereupon he adopted the name of Motomasa.  It was determined that, in the future, he would wed Motosato’s daughter and inherit the Asonuma clan.  It was further decided that if the covenant was breached, then Motosato’s fief would be granted to Motosato’s daughter.

On 4/29 of Keichō 14 (1609), his natural father died and, on 12/28, Motomasa was conferred by Terumoto the title of Lieutenant of Outer Palace Guards of the Left Division.  On 10/10 of Keichō 15 (1610), Motomasa was granted by Terumoto a written pledge sealed in blood stating there was no intent toward estrangement between them.  On 11/19 of Keichō 17 (1612), Mōri Hidenari granted Motomasa the official title of 帯刀 Lieutenant of Outer Palace Guards of the Left Division and, on 9/5 of Kanei 9 (1632), the honorary title of Governor of Iwami.

On 3/3 of Kanei 14 (1637), Hidenari recognized the succession by Motomasa’s son, 就致, of his fief of 2,500 koku.  Motomasa died on 11/14 of Jōou 2 (1653).