- Born: 1544
- Died: 1611
- Title: Awa no Kami 安房守
- “Japanese:” 真田昌幸 “(Sanada Masayuki)”
- Distinction: considered one of the most skilled samurai commanders of the later Sengoku Period; sometimes considered one of Takeda Shingen's 24 Generals
Masayuki was the 3rd son of Sanada Yukitaka (1512-1574) and like his father first served Takeda Shingen. His first taste of battle, in fact, is said to have been at the famous Fourth Battle of Kawanakajima. At one point he was known as Mutô Kibêjô 武藤喜兵衛尉, but his two elder brothers, Nobutsuna and Masateru, were killed in 1575 at Nagashino, and he became head of the Sanada.
As the power of the Takeda declined, Masayuki expanded from northern Shinano into Kôzuke in 1580 and took [[Numata castle] from the Hôjô, who controlled most of the Kantô. Numata is in northern Kôzuke on the Tone River 利根川, which flows into the Kantô plain, so the Sanada had access to all three highways between Kantô and Echigo province.
In 1582 Oda Nobunaga attacked the Takeda, and as their cause was hopeless, Masayuki wrote the Hôjô requesting to become their vassal and after Nobunaga’s death, sent hostages to them. But Shinano was semi-controlled in quick succession by various warlords, and Masayuki accepted whoever was in control. He ended up serving Tokugawa Ieyasu, who now ruled the former Takeda center of Kai.
Masayuki now worked on consolidating control of the Chiisagata district of Shinano. He had built a number of mountain castles as Sanada-yama castle and Matsuo castle and still held on to Numata castle, but in 1583 started the construction of the flatland Ueda castle on the junction of the Tôsendô 東山道 (Nakasendô) and the Hokkoku Highway 北国街道.
In 1585 Ieyasu demanded that Numata be returned to the Hôjô as part of a Tokugawa-Hôjô agreement signed that year. Masayuki refused, allied with Uesugi Kagekatsu of Echigo, and sent his second son Nobushige as a hostage to Kaizu in the 8th month in return for assistance at Numata against the Hôjô. In the intercalary 8th month (int. 8) he resisted an attack on Ueda by a Tokugawa army sent to chastise him. Also, the Hôjô failed to bring down Numata, which was held by Masayuki's uncle, Yasawa Yoritsuna. Masayuki asked Hashiba (Toyotomi) Hideyoshi to intervene  and finally made peace with the Tokugawa by sending his elder son Nobuyuki as a hostage to Hamamatsu.
In 1589 Hideyoshi arbitrated the dispute over Numata, giving it to the Hôjô, but let the Sanada keep Nagurumi castle and the Azuma district of Kôzuke, and also gave them part of the Ina district of Shinano as a replacement for Numata. However, in the eleventh month the Hôjô commander in Numata attacked and took Nagurumi castle. Hideyoshi immediately announced he would attack the Hôjô, and the next year carried out the Odawara Campaign. Masayuki participated by attacking Hôjô forts in Kôzuke and saw his territories increased somewhat as a result. Numata was given to his elder son Nobuyuki.
In 1600 during the Sekigahara campaign Masayuki and his two elder sons accompanied the Tokugawa on their march against Uesugi Kagekatsu, but Ishida Mitsunari’s call to arms of 7/17 reached them in Shimotsuke province. Masayuki was furious that he had not been informed beforehand,  but he and Nobushige declared for the 'western' cause and returned to Ueda, though Nobuyuki joined the Tokugawa camp, probably with his father’s encouragement to preserve the family whichever side won. Masayuki and Nobushige were besieged in Ueda by Tokugawa Hidetada but succesfully resisted the Eastern forces, doing so in such a staunch manner that Ueda is considered one of the 'classic' sieges of Japanese history.
Having won control of the country, at the end of the year, Tokugawa Ieyasu banished both Masayuki and Yukimura to Kudoyama of Kôyazan in Kai province. Nobuyuki, who had been given Ueda, supported them, but Masayuki in his lettters often complained of debt. Apparently thought he would soon be pardoned, but that was not to be, and he died in 1611.
Yukimura would become the major figure on the Toyotomi side during the - Osaka Campaign, while Nobuyuki, who was later transfered to Matsushiro in NW Shinano, would live into his 90's, and his descendents ruled there until the Restoration.
Masayuki is considered one of the most skilled samurai commanders of the later Sengoku Period, and foul play has traditionally been suspected in his death.
- Letter from Hôjô Ujikuni to Masayuki dated 1582/3/12 (Sanada Family Materials, p. 57).
- Letter from Hideyoshi to Masayuki dated 1585/10/17 (Sanada Family Materials, p. 84).
- Letter from Mitsunari to Masayuki dated 1600/7/30 (Sanada Family Materials, p. 94).
- Letter of Masayuki dated 1603/3/15 (Sanada Family Materials, p. 99).
- Initial text from Samurai-Archives.com FWSeal & CEWest, 2005