Lifespan: Keichō 7 (1602) to 1/4 of Kanei 4 (1627)
Rank: Second head of Mutsu-Aizu domain
Title: Junior Third Rank and Councillor, Senior Fourth Rank and Councillor, Governor of Shimotsuke
Lord: Tokugawa Hidetada → Tokugawa Iemitsu
Father: Gamō Hideyuki
Mother: Furihime (third daughter of Tokugawa Ieyasu)
Siblings: Tadasato, Tadatomo, Sūhōin (wife of Katō Tadahiro), Asano Mitsuakira (younger brother of a different father)
Wife: [Formal] Daughter of Tōdō Takatora
In the fifth month of 1612, Hideyuki suddenly died at the age of thirty so, at the age of ten, Tadasato inherited the Aizu domain with 600,000 koku. In the ninth month, Tadasato and his younger brother, Tsuruchiyo (later known as Tadatomo), attended a coming-of-age ceremony (sponsored by Tokugawa Ieyasu) and received the Matsudaira surname along with one of the characters from the name of Tokugawa Hidetada, the shōgun. He also received one of the characters from the name of his grandfather, Gamō Ujisato, so he adopted the name of Matsudaira Tadasato. However, owing to his youth, his mother, Furihime, served as his guardian.
Dating back to the era of Ujisato, the Gamō family experienced internal tensions that continued during the period that Tadasato served as head of the domain. Following the death of Hideyuki, Furihime soon came into a severe conflict with the chief retainer of the Gamō clan named Oka Shigemasa over matters of governance of the domain. This culminated in Tokugawa Ieyasu summoning Shigemasa to Sunpu Castle and ordering him to commit seppuku in 1613. Owing to this instability, during the Siege of Ōsaka in 1615, Tadasato was ordered to remain in the Gamō residence in Edo.
During his term, Tadasato arranged for the reconstruction of a five-story tower at Aizu-Wakamatsu Castle and provided support for the rebuilding of the Eryū Temple in Aizu. During a period of starvation in his domain, he ordered a prohibition against the production of mochi, saké, and tōfu.
In 1619, Tadasato received the daughter of Tōdō Takatora as his formal wife. In 1623, he was appointed a chamberlain. In 1624, he received Tokugawa Hidetada (the Ōgosho) and Tokugawa Iemitsu (the shōgun) at the Gamō residence in Edo. During this period, conflicts and claims persisted among his senior retainers, but these were regarded as concerns of the clan to resolve. In 1626, upon the occasion of a visit by Emperor Gomizu-no-o to the Nijō Castle, Tadasato went to Kyōto and, after the Imperial visit, was promoted to Senior Fourth Rank (Lower) and Councillor. At this time, he contracted smallpox and, on 1/4 of Kanei 4 (1627) died at the age of twenty-six.
Tadasato did not produce an heir with his formal wife, so, the lineage of the Gamō clan should have come to an end. Nevertheless, his mother was the daughter of Tokugawa Ieyasu, so his younger brother, Gamō Tadatomo (who was the head of the Dewa-Kami-no-yama domain with a fief of 40,000 koku) was appointed as Tadasato’s heir and given a fief of 240,000 koku in Iyo-Matsuyama in Shikoku. Although the fief of the Gamō family was reduced from 600,000 koku in Aizu to 240,000 koku in Matsuyama, their lineage was preserved. Katō Yoshiaki then entered Aizu in lieu of the Gamō with a fief of 400,000 koku.