Maruko Sanzaemon served as a bushō during the Sengoku and Azuchi-Momoyama periods. He was a retainer of the Sanada clan and served as the lord of Maruko Castle near Ueda in Shinano Province. The Maruko clan were a small-scale gōzoku, or wealthy family, based at Maruko Castle near Ueda.
In Tenshō 13 (1585), during the First Battle of Ueda, the Tokugawa launched a failed assault against Ueda Castle. On 8/2, Tokugawa forces clashed with the Sanada army near the Kami River (Kamikawa) but were repelled. Under another account, the Tokugawa assaulted Ueda Castle, penetrating to the outer citadel, but retreated in the face of a counterattack by the defenders. During the retreat, the Tokugawa were pursued by the Sanada forces while Sanada Nobuyuki led an attack from the flank, causing the forces to fall apart. Yazawa forces further joined in the clashes, with many commanders and troops from the Tokugawa drowning in the Kami River. By exploiting their knowledge of the local terrain, the Sanada killed as many as 1,300 members of the Tokugawa army while losing only 40 troops of their own.
The next day, the Tokugawa forces turned their attention to neighboring Maruko Castle defended by Sanzaemon with a small garrison. The Tokugawa set-up a camp at Yaehara to the east of Maruko Castle. Sanada Masayuki went to Teshiozuka to counter them. On 8/19, Suwa Yoritada commenced an assault on Maruko Castle. On 8/20, Masayuki went to Nagasekawahara and assaulted the Suwa army from behind with arquebus fire. A retainer of the Tokugawa named Okabe Nagamori responded by dividing the forces into three units to mount an attack, setting fire to the town of Kawahara and repelling the Sanada army. His unit then attacked Maruko Castle. Owing, however, to stiff resistance by the defenders, the Tokugawa forces were unable to capture the castle. This event is known as the Battle of Maruko-omote. After this battle, Ieyasu gave letters of commendation to seventeen subordinates, suggesting that it was a robust battle.
Later, Sanzaemon was granted the Unno surname and served Sanada Nobuyuki.