Lifespan: Tenshō 19 (1591) to the seventh month of Manji 3 (1660)
Domain: Hizen-Karatsu → Musashi-Kawagoe → Shinano-Matsumoto → Shimōsa-Sakura
Lord: Chōsokabe Motochika → Chōsokabe Morichika → Terazawa Hirotaka → Hotta Masamori → Hotta Masanobu
Father: Kōsokabe Chikayasu
Mother: Daughter of Kōsokabe Hidemichi
Siblings: Chikauji, Sadachika
Adopted Children: Shigechika (natural son of Takai Genzaemon)
Kōsokabe Sadachika served as a bushō during the Azuchi-Momoyama and early Edo periods. He was the head of the Kōsokabe clan.
In 1591, Sadachika was born as the second son of Kōsokabe Chikayasu, a senior retainer of the Chōsokabe clan. His childhood name was Chōjumaru. He was also known as Chikakazu. His father and older brother died one after another so, in 1593, Sadachika inherited the headship of the clan at the age of three. At the Battle of Sekigahara, after his lord was removed from his position, Sadachika became a rōnin, or masterless samurai. He left Tosa Province and moved to Sakai in Izumi Province. Later, he served Terazawa Hirotaka, the lord of the Hizen-Karatsu domain and received a fief of 500 koku. At the Siege of Ōsaka, his cousin, Chōsokabe Morichika joined the Toyotomi, so Sadachika changed his name to Nakahara Kizaemon.
Later, he left the Terazawa family and became a rōnin again, but, through the offices of Kasuga-no-tsubone, Kizaemon was engaged by Hotta Masamori, the lord of the Musashi-Kawagoe domain and granted a fief of 1,000 koku that was later increased to 1,300 koku. He transferred to the Shinano-Matsumoto domain and Shimōsa-Sakura domain of the Hotta family, and died in the seventh month of Manji 3 (1660). Authenticated accounts of the Kōsokabe family were written by Kizaemon.
Kizaemon was succeeded by his adopted son, Shigechika (the natural son of Takai Genzaemon, a retainer of the Hotta clan). In the tenth month of 1660, after the removal of the Sakura domain, Shigechika went, with the assistance of Igarashi Motonari and Shibata Tomomoto (siblings whose mother was a cousin of Kizaemon), to Sendai. Shigechika was engaged by the Sendai domain with a fief of 2,000 koku. He then served the Hotta family of the Koga domain.