Tatsuoki was the son of Saitô Yoshitatsu and his mother was a daughter of Asai Hisamasa. He succeeded his father when the latter died of illness in 1561. He inherited a war with the Oda and saw his forces defeated in a series of engagements that culminated in the Battle of Moribe in 1561. He was unable to prevent the Oda from penetrating Mino and also had to contend with the Asai on his western border. Nobunaga managed to lure away the Saitô's top generals (including Ando Morinari, Inaba Ittetsu, and Ujiie Bokuzen) and thus considerably weakened Tatsuoki's position. In 1567 Nobunaga captured Inokuchi (Inabayama) and Tatsuoki was sent off into exile and took up with the Asakura of Echizen. He was killed at Tonezaka when the Oda invaded Echizen in 1573.
- Initial text from Sengoku Biographical Dictionary (Samurai-Archives.com) FWSeal & CEWest, 2005