Lifespan: Tenbun 5 (1536) to 5/23 of Tenshō 5 (1577)
Other Names: Kotarō, Natsukasa-no-jō
Lord: Chiba clan → Hōjō clan → Satomi clan
Father: Takeuchi Izumo-no-kami
Mother: Daughter of Sakai 玄治
Sakai Taneharu served as a bushō during the Sengoku and Azuchi-Momoyama periods. He was the lord of Toke Castle in Kazusa Province.
Based on the genealogy, Taneharu was born as the son of Sakai 玄治, but, in fact, he is regarded as the son of the daughter of 玄治 named Myōgei and a retainer of the Sakai named Takeuchi Izumo-no-kami. During the early part of the Tenbun era (1532 to 1555), an internal conflict arose in the Kazusa-Sakai clan with one faction supporting the Koga kubō and the other faction backing the Oyumi kubō. In the course of these events, Taneharu (who was either from an illegitimate branch who inherited the bloodline of the Toke-Sakai clan via his mother or originated from the lineage of a retainer) is surmised to have received backing as the lord of Toke Castle in lieu of the lineal heirs of the Toke-Sakai clan who were ousted during the family conflict. His name first appears in a historical artifact on the inscription of a statue of Nichiren (the founder of the Hokke sect) built in 1546 and located at the Honkō Temple in Kamakura in which he is referred to as a parishioner named Sakai Kotarō.
Initially, Taneharu served as a senior retainer of the Chiba clan under the command of Hara Tanekiyo of Oyumi Castle. After the Chiba came under the influence of the Hōjō, Taneharu was folded into the group of retainers from other provinces serving the Hōjō and engaged in assorted battles. In 1549, Hōjō Ujiyasu granted the Ninomiya manor in Kazusa to Taneharu. His eldest son, Sakai Masashige (who died early) is deemed to have received one of the characters in his name from Hōjō Ujimasa. In 1561, during an assault by Masaki Tokishige, the Tōgane-Sakai clan surrendered but Taneharu remained completely loyal to the Hōjō to the end. In 1559, he mobilized members and retainers of the Tōgane-Sakai clan to renovate the Honkō Temple which had ties to Sakai Sadataka, the founder of the Kazusa-Sakai clan. On the sign staked to the temple’s ridgepole at construction time stating the names of donors, the builder, the date, and the reason for construction, he is listed as a key parishioner under the name of Sakai Nakatsukasa-no-jō Taneharu.
In the first month of 1564, sudden orders to mobilize for the Second Battle of Kōnodai at the start of the new year caused delay, during which Taneharu heard a report that Hōjō Ujiyasu began to distrust whether Taneharu would deploy. Taneharu objected to this accusation and then proceeded to collude with the Satomi clan, abandoning the Hōjō and providing assistance to Satomi Yoshihiro who was fleeing in defeat.
As Toke Castle at once became the front line on the Bōsō Peninsula for the Satomi, the Hōjō army attacked on a nearly annual basis. Taneharu, with the support of Uesugi Kenshin and Satomi Yoshihiro, was able to repel these offensives by the Hōjō. Owing, however, to the alignment of Sakai Tanetoshi of Tōgane Castle with the Hōjō along with the utter destruction wrought by the Hōjō army, in 1576, Taneharu was finally compelled to tender his son, Sakai Yasuharu, as a hostage and surrender. Before long, Yasuharu was released and, after transferring the headship of the clan to him, Taneharu retired. According to other accounts, as of 1568, Yasuharu had already become the head of the clan so there is a possibility that Taneharu had been forced to retire that same year.