Sanponji Sadanaga served as a bushō during the Sengoku and Azuchi-Momoyama period.  He was a retainer of the Uesugi clan and head of the Sanponji-Uesugi family.  He served as the lord of Fudōsan Castle.

The Sanponji-Uesugi were an illegitimate branch of the Uesugi clan.  According to military service records from 1575, the family was ranked sixth as members of the Uesugi clan.

In 1519, Sadanaga was born as the son of Sanponji Sadakage.  His mother appears to have been the daughter of Nagao Yoshikage.  He was the grandson of Sanponji Sadatane who martyred himself for Uesugi Fusayoshi.

Sadanaga quarreled with the Nagao clan but followed Nagao Harukage who backed Uesugi Sadazane to serve as a nominal military governor.  In 1555, Sadanaga served valorously as a retainer of Uesugi Kenshin in the Second Battle of Kawanakajima.  Later, he was dispatched to the front lines in Etchū Province.  In 1572, he deployed in support of Hinomiya Castle in Etchū after it was surrounded by Ikkō-ikki forces.  He was then defeated in battle at a landing to the Jinzū River.

After Uesugi Kagetora (Hōjō Saburō) was adopted by Kenshin, Sadanaga was appointed to serve as his mentor.  Through this connection, in 1578, he sided with Kagetora in the Otate Conflict and was defeated.  He abandoned his home and absconded.  The headship of the family was inherited by Sanponji Kagenaga who was either his son or younger brother.