Lifespan: Eiroku 2 (1559) to 4/29 of Keichō 14 (1609)
Title: Junior Fifth Rank (Lower), Director of the Bureau of Palace Maintenance, Governor of Sanuki
Clan: Ōe-Mōri → Fujiwara-Amano → Tatara-Migita → Mōri
Lord: Mōri Terumoto
Father: Mōri Motonari
Adoptive Fathers: Amano Motosada, Migita Takakazu
Adoptive Mother: Daughter of Asonuma Hirohide
Siblings: Mōri Takamoto, Kikkawa Motoharu, Kobayakawa Takakage, Ninomiya Naritoki, Hoida Motokiyo, Mōri Motoaki, Izuha Mototomo, Motomasa, Suetsugu Motoyasu, Mōri Hidekane, others
Wives: [Formal] Daughter of Amano Motosada, [Second] Daughter of Asonuma Hirohide, daughter of Kinashi Takamori, daughter of the Nagashima clan
Children: Mōri Mototomo, Amano Motochika, Asonuma Motomasa, Amano Motomasa, Amano 就員, Amano Motoyoshi, daughter (formal wife of Mōri Motonobu), daughter (wife of Hosokawa Mototō), daughter (wife of Suginomori Motochika), daughter
Amano Motomasa served as a bushō during the Sengoku and early Edo periods. He was a retainer of the Mōri and founder of the Migita-Mōri family.
In 1559, Motomasa was born as the seventh son of Mōri Motonari, the sengoku daimyō of Aki Province. His mother was Nomi-no-ōkata. Hoida Motokiyo and Mōri Hidekane were brothers of the same parents. The Fukino-Amano clan, similar to the neighboring Kinmeizan-Amano clan, was a kokujin, or provincial landowner, in Aki Province founded by Amano Tōkage in the Kamakura period. In 1569, following the death of Amano Motosada (the head of the clan) an internal dispute arose over his succession. This prompted Motonari to intervene and have his seventh son, Chitoramaru (later known as Motomasa) as a son-in-law taken into the family of Motosada to become the next head of the clan, resolving the situation. Thereafter, he became the adopted son of Migita Takakazu, the head of the Migita clan (a branch of the Ōuchi clan) and received the family name.
As a family member of the Mōri, Motomasa deployed for many battles. At the Siege of Kōzuki Castle from 4/18 to 7/3 of 1578, he joined his brother, Motokiyo, as a commander of the army and succeeded in toppling Kōzuki Castle. Thereafter, he operated as a key member of the Mōri clan, and, in 1596, was given the Toyotomi surname by Toyotomi Hideyoshi. Motomasa also participated in the Bunroku-Keichō Campaign and, in 1600, at the Battle of Sekigahara.
Later, he reverted to the Mōri surname and, in 1603, was transferred to Mitsuo in the Kumage District of Suō Province. Motomasa received one of the teeth that was lost by Motonari and carried it close to his person. In the year marking the thirty-third anniversary of the death of Motonari (a traditional Buddhist custom) he built a memorial tower and placed the tooth inside. He was later awarded a fief of 13,000 koku in the Migita territory. Although he resumed use of the Mōri surname, he gave the Amano surname to his sons, Motomasa and Motoyoshi. In 1605, when a dispute arose in the family in regard to the construction of Hagi Castle along the coast in Suō, Motomasa surveyed the opinions of Amano Motonubu and Masuda Motonaga and reported to his lord, Mōri Terumoto.
Motomasa died on 4/29 of 1609 in Hagi at the age of fifty-one.