The surname of Sumitomo originates from Kotaro (Tadashige), the twenty-second-generation descendant of Takamochi-oh, a great grandson of Emperor Kanmu, who took the surname "Sumitomo" after the surname and given name of his father, Masami Hirauchi Tomosada, who lived at the end of the Sengoku period; and Kotaro served Shogun Muromachi and was appointed as the Bichumori guard.
住友の姓は、戦国の末、もともと先祖に順美平内友定という人物がおり、桓武天皇の曾孫・高望王の二十二代目にその子・小太郎（忠重）が父の姓と名をとって「住友」の姓を称して室町将軍に仕えて、備中守に任じられたのに始まる。