Higashi Naomasa served as a bushō during the Sengoku period. He was a retainer of the Sagara clan of Higo Province and served as the lord of Yunomae Castle.
In 1559, there was a falling out in the relationship between Marume Yoriyoshi and Higashi Nagatoshi – two magistrates serving in Hitoyoshi on behalf of the Sagara clan. At this time, perhaps because Naomasa was a relative of Yoriyoshi, he was called by Naijō (the mother of Sagara Yoshihi, a sengoku daimyō and the eighteenth head of the Sagara clan) to Hitoyoshi Castle whereupon she requested that he mediate the dispute between the magistrates. Initially, Naomasa expected that he would be ordered to persuade Yoriyoshi or else to execute him, but neither order was given so he went to the residence of Yoriyoshi and promised to align with him. In the fifth month of the same year, Nagatoshi garnered support from Yoshihi and Naijō to attack the residence of Yoriyoshi, causing Yoriyoshi to flee with his wife and children to Yunomae Castle. While Naomasa was opposed to Yoshihi, he did not intend to align with him, but after being persuaded by his guardian, Ueda Heibu, he joined those holing-up in the castle.
On the evening of 8/15, in an effort to capture Taragi Castle, Naomasa assigned soldiers to his younger brother, Tsunematsu Kurōdo, and had them deploy. Kurōdo and his men proceeded to decimate troops from the Taragi group laying in ambush to attack them but were unable to topple Taragi Castle. The next day, he acquired backing from Nasu Suō-no-kami and Nasu Hyōbu-dayū of the Shiiba District in Hyūga and violently clashed with the Sagara forces at Usonobaru. Initially, the Taragi group broke-through the enemy formation, gaining the upper hand, but were then pushed by forces from Hitoyoshi led by Indō Yoriyasu. Amidst the mayhem, Yoriyoshi fled to Hyūga while Naomasa, along with Kurōdo and over 180 soldiers, were killed in action. This event is known as the Battle of Usonobaru.