Yamaguchi Shigemasa served as a bushō and daimyō during the Azuchi-Momoyama and early Edo periods.  He was the lord of Hoshizaki Castle in Owari Province.  Shigemasa served as the first lord of the Hitachi-Ushiku domain.

Shigemasa was a member of the Ushiku-Yamaguchi clan, an illegitimate branch of the Ōuchi clan based in the western provinces.  The Yamaguchi surname draws from the base of the Ōuchi at Yamaguchi in Suo Province.  Shigemasa, however, resided in Owari and became a retainer of the Oda clan.

In 1564, Shigemasa was born in Owari and the eldest son of Yamaguchi Morimasa.  He was adopted by Yamaguchi Shigekatsu, the lord of Terabe Castle in Owari.

Originally, he served under Sakuma Masakatsu, a retainer of Oda Nobukatsu.  In 1584, Shigemasa participated in the Battle of Komaki-Nagakute as a member of the allied army of the Oda and Tokugawa.  After the fall in succession of Shimoichiba, Maeda, and Kanie castles in Owari, Shigemasa fought desperately against an assault by Takigawa Kazumasu (on the side of the Toyotomi) to defend Ōno Castle.  This is known as the Siege of Kanie Castle.  In 1586, he inherited the headship of the clan.

In 1590, his lord, Oda Nobukatsu, refused orders from Toyotomi Hideyoshi to transfer to Suruga Province so Nobukatsu was removed from his position.  Shigemasa then followed Nobukatsu to Shimotsuke Province.  Later, he became a retainer of Tokugawa Hidetada and received an income of 5,000 koku.  In 1600, at the Battle of Sekigahara, Shigemasa served in Hidetada’s army, participating in an assault against Sanada Masayuki at Ueda Castle in Shinano Province.  Owing to these contributions, Shigemasa became a daimyō with a fief of 10,000 koku in Hitachi Province.  In 1606, he was appointed as the ōbantō, or the official in charge of collecting annual levies from groups of individuals (formed for tasks or projects requiring more than one person) treated as a single unit for tax purposes.

In 1613, however, Shigemasa was criticized for allowing his eldest son, Yamaguchi Shigenobu, to wed the adopted daughter of Ōkubo Tadachika, without the permission of the bakufu, so he was removed from his position and ordered into confinement at the Ryūon Temple in Musashi Province.  After his confinement, Shigemasa was extraordinarily committed to a revival of the Yamaguchi family.  In 1614, after the start of the Winter Campaign of the Siege of Ōsaka, Shigemasa proposed to Tokugawa Ieyasu that Shigemasa and others would join the Toyotomi clan, and after assassinating Toyotomi Hideyori, would ask for permission to revive his family as compensation for the deed.  Ieyasu, however, declined to accept his offer.  In 1615, during the Summer Campaign of the Siege of Ōsaka, Shigemasa served in the army of Ii Naotaka and fought valiantly at the Battle of Wakae, but, after advancing with his son, Shigenobu, too far forward, was killed by Kimura Shigenari.  Shigemasa’s younger brother, Yamaguchi Shigekatsu, also died in battle.

In later years, based on his contributions, in 1628, Shigemasa reclaimed his status as a daimyō with a fief of 15,000 koku in Hitachi-Ushiku.  He was also appointed as a sōjaban, an official role responsible for managing ceremonial matters for military families residing in castles.  There is a story that, at this time, the descendants of Kimura Shigenari were engaged by the Ushiku domain.  On 9/19 of Kanei 12 (1635), he died at the age of seventy-two.

His descendants continued as members of the Hitachi-Ushiku domain and some as retainers of the Hitachi-Mito domain.

The Ushiku-Yamaguchi clan used the character of “Hiro” for generations.  This includes the names of Yamaguchi Hirosada, the husband of the younger sister of Kimura Shigenari who was killed fighting against Ii forces at the Battle of Yao and Wakae, and his older brother, Ukyō-no-suke Yamaguchi Nagahiro, who was killed at the Daishōji Castle in Kaga Province during the Battle of Sekigahara.  If they were members of the same family, then Shigemasa was an indirect relative of Kimura Shigenari.