Morimasa was the son of Sakuma Moritsugu, a cousin of Sakuma Nobumori, and his mother was Shibata Katsuie's elder sister. He became one of Shibata Katsuie's top retainers and accompanied him on most of his campaigns while serving the Oda. He was established at Oyama in Kaga province and owing to his aggressive spirit was nicknamed Onigenba. He led Shibata's army into northern Ômi in 1583 when war came with Toyotomi Hideyoshi. In the course of the campaign he captured Iwasakiyama from Takayama Ukon and was besieging Shizugatake when Katsuie ordered him to withdraw lest Hideyoshi catch him off-guard. Sakuma, however, over-estimated the time it would take Hideyoshi to arrive on the scene and so disobeyed Katsuie. Hideyoshi in fact reacted swiftly and in the resulting Battle of Shizugatake, Sakuma's army was routed. Morimasa was captured and later beheaded in Kyoto.
- Initial text from Sengoku Biographical Dictionary (Samurai-Archives.com) FWSeal & CEWest, 2005