Bessho Nagaharu served as a bushō and daimyō during the Sengoku and Azuchi-Momoyama periods.

Nagaharu was the eldest son of Bessho Yasuharu.  In 1570, Nagaharu became head of the clan following the death by illness of Yasuharu.  Owing to his young age, Nagaharu served under the guardianship of his uncles, Bessho Yoshichika and Bessho Shigemune.  The members of the Bessho clan were longstanding retainers of Oda Nobunaga.  In the tenth month of 1575, Nagaharu paid a visit to Nobunaga, and visited the following year for new year’s greetings.  In 1577, Nagaharu joined the deployment for the Conquest of Kishū (Kii Province).  Nagaharu then served in the vanguard of the campaign against the Mōri in the Chūgoku region.  However, after witnessing a slaughter at Kōzuki Castle in Harima Province, along with dissatisfaction with the actions of the rising Hashiba Hideyoshi, supreme commander of the forces in the Chūgoku region, Nagaharu opted for an alternate path.  He acted in concert with Hatano Hideharu of Tanba Province (the home province of his wife) to rebel against Nobunaga.  Many surrounding clans supported the action and attacked those who did not abide so that eastern Harima became unified in its opposition to the Oda.

Consequently, Nobunaga ordered Hideyoshi to attack Nagaharu.  Nagaharu made a stiff defense from Miki Castle, causing trouble for Hideyoshi.  Meanwhile, Araki Murashige rebelled against Nobunaga and the Mōri dispatched reinforcements, leading to a temporary withdrawal by the Oda forces.  Nagaharu even went on the offensive, pinning Reizei Tamezumi and his son in Ureshino Castle and causing them to take their own lives.  Finally, the confrontation led to the Siege of Miki whereby Hideyoshi deployed a tactic to wait-out the defenders, knowing that their supplies would eventually run out. First, the Oda captured the outlying castles of Kanki and Shikata.  Reinforcements from the Mōri were blocked.  Then, two years later, in 1580, Nagaharu and his family offered to kill themselves in exchange for sparing the lives of the defenders.  Prior to this act, Hideyoshi allowed for a final ceremonial dinner.  His retainer, Miyake Harutada, served in the honorary role as the executioner.  Bessho Yoshiharu, the eldest son and heir of Bessho Shigemune, became a daimyō, serving as the head of the Yagi domain of Tajima Province in the early Edo period.  Under another theory, Yoshiharu was the son of Nagaharu, and, upon the fall of Miki Castle, was taken by retainers to Shigemune who raised him.

Under another account, a retainer named Gotō Motokuni (the father of Gotō Mototsugu) enabled the escape by an eight-year-old son of Nagaharu named Chiyomaru, together with his caretaker and servants to Kōzu Castle and, after the fall of that castle, Chiyomaru engaged in farming.