Lifespan: Unknown to Tenshō 6 (1578)
Other Names: Kiunsai (monk’s name)
Title: Sagami-no-kami, or Governor of Sagami (informal)
Lord: Itō Yoshisuke
Father: Itō Sukemune
Children: Sukemune (written with same characters as his grandfather), Sukemoto
Itō Sukemasu served as a bushō during the Sengoku and Azuchi-Momoyama periods. He was a retainer of the Itō clan of Hyūga Province.
In 1533, after the demise of Itō Sukemitsu, a sengoku daimyō and the ninth head of the Hyūga-Itō, Sukemasu’s uncle, Itō Suketake, launched a rebellion. At this time, Sukemasu backed his cousin, Itō Sukekiyo (Yoshisuke), as the successor. After the death of Itō Sukeyoshi, the tenth head of the clan, and succession by Yoshisuke, Sukemasu served as his close associate. Within the family, Sukemasu wielded authority on a par with Yoshisuke; however, by misappropriating excess quantities of rice, along with other acts, he stirred resentment among the retainers.
In 1572, in the course of the defeat of the Itō at the Battle of Kizakibaru, Sukemasu’s grandson, Itō Sukenobu, was killed in action. That same year, Sukemasu’s son, Itō Sukemune, died while serving as a guardian for Itō Suketake at Obi Castle.
In 1575, based on a proposal from Kimotsuki Kaneaki, a daimyō and the eighteenth head of the Kimotsuki clan under pressure from the Shimazu, the Itō and the Kimotsuki planned to stage a conflict with empty guns, but the Itō broke the promise and used real ammunition so the Kimotsuki were decimated and both families disowned. In this incident, Sukemasu plotted to capture Nangō in the territory of the Kimotsuki.
Owing to personal enmity toward Sukemasu, during the period of decline of the Itō clan, many retainers including Mera Norishige and Ochiai Kanetomo, betrayed the Itō in favor of the Shimazu clan.
After the fall of the main branch of the family, Sukemasu followed Yoshisuke in retreat to Bungo Province. In 1578, Sukemasu died in Sanjō in Hyūga.