Sanponji Kagenaga served as a bushō during the Sengoku period.  He was the head of the Sanponji-Uesugi family and the lord of Fudōsan Castle.

The Sanponji-Uesugi family were an illegitimate branch of the Uesugi clan serving as the military governors of Echigo Province.

Kagenaga was born as the son of Sanponji Sadanaga.  Under another theory, Kagenaga was the younger brother of Sadanaga.  In 1578, during a succession struggle in the Uesugi clan known as the Otate Conflict, Sadanaga sided with Uesugi Kagetora and, after a defeat, was banished.  Kagenaga succeeded him as the head of the family.

In the fourth month of 1582, Shibata Katsuie (a senior retainer of Oda Nobunaga) led 40,000 troops in an invasion of Echigo during a period of conflict within the Uesugi clan.  At this time, Kagenaga joined Nakajō Kageyasu, Takenomata Yoshitsuna, and Yoshie Munenobu to hole-up in Uozu Castle in Etchū Province.  This is known as the Siege of Uozu Castle.  The Uesugi clan had consumed military resources during the Otate Conflict.  Moreover, their forces were occupied in a stand-off with the Oda army in the direction of Shinano Province so reinforcements were not available to send to Uozu Castle.  As a result, Uesugi Kagekatsu permitted them to surrender but Kagenaga did not open the castle and, after holding out, took his own life.  This occurred early in the sixth month, on the day after the coup d’état against Nobunaga in Kyōto, an event known as the Honnō Temple Incident.

Although Kagenaga’s year of birth is unknown, it is surmised that he was young.  His orphan later entered the priesthood, becoming the eighteenth high priest at the Rinsen Temple in Echigo.

During the Otate Conflict, his younger brother (or son) named Tōsaburō Katsunaga, sided with Kagetora and, after the defeat, lived in seclusion but later returned to the service of the Uesugi family.  In 1594, around the time that Naoe Kanetsugu served as the magistrate overseeing the construction of Fushimi Castle, he was removed from his position.  In 1637, he returned to service for a stipend of 300 koku (later 500 koku) to manage the conduct of ceremonial affairs for the Yonezawa domain.