A court noble and warrior of the mid-Heian period. Born into the Fujiwara clan, he was posted as an official in Iyo Province (present-day Ehime Prefecture) before organizing and leading a pirate confederation in the Seto Inland Sea into rebellion (the Jōhei-Tengyō Rebellion, 939–941). His uprising, paired with that of Taira no Masakado in the east, is known collectively as the "Masakado-Sumitomo Rebellion" and starkly exposed the court's dwindling reach over the provinces. Sumitomo at one point captured the Dazaifu headquarters in Kyushu and established a vast sphere of control over western Japan. The court dispatched Ono no Yoshifuru and Minamoto no Tsunemoto to suppress him; Sumitomo was defeated in 941 and his severed head was sent to the capital. The rebellion's background is traced to the contradictions of aristocratic society and corruption in provincial administration, and some historians view Sumitomo as a voice for the people of the regions.

A singular figure who, despite aristocratic origins, became a pirate and rebel. He chafed at the constraints of central authority and loved the sea and freedom. He unified a diverse confederation of pirates through organizational skill and strategic talent, briefly establishing an independent sphere in western Japan.

The Masakado-Sumitomo Rebellion stands as a symbolic turning point in Japanese history, marking the rise of the warrior class. The pirate culture of the Seto Inland Sea was inherited by later maritime forces such as the Murakami Navy, carrying on the tradition of free people of the sea. Legends associated with Sumitomo remain along the coasts of Ehime and Hiroshima Prefectures.