Nagao Sadakage served as a bushi during the late Muromachi period.  He was the second head of the Ashikaga-Nagao clan, retainers of the Yamanouchi-Uesugi family.

Sadakage was born as the lineal heir of Nagao Kagehito.  In 1472, after the death of his father, Sadakage inherited the headship of the clan.  He received one of the characters from the name of his lord, Uesugi Akisada (the deputy shōgun of the Kantō), and adopted the name of Sadakage.  Owing to frailness of health, he suddenly died just three years later.

As a result, his younger brother, Nagao Kagenaga, succeeded him under the guardianship of his uncle, Nagao Fusakiyo.