Sakai Tanetoshi served as a bushō during the Sengoku and Azuchi-Momoyama periods. He was the lord of Tōgane Castle in Kazusa Province.
Initially, Tanetoshi 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, Tanetoshi was folded into the group of retainers from other provinces serving the Hōjō and engaged in assorted battles. It appears that he changed his name from Toshifusa to Tanetoshi after receiving one of the characters in his name from the Hara clan of Shimōsa Province.
In 1561, during an assault by Masaki Tokishige, Tōgane Castle fell and the Tōgane-Sakai clan surrendered and moved to Shiizaki Castle in the Musa District of Kazusa. Later that year, members of the Tōgane-Sakai deployed to the Kantō in obedience to Uesugi Kenshin. The clan then reverted to the service of the Hōjō and, in 1564, at the Second Battle of Kōnodai, fought on behalf of the Hōjō and reclaimed Tōgane Castle. Tanetoshi frequently clashed with Sakai Taneharu, a member of the same family as Tanetoshi and serving as the lord of Toke Castle. During the foregoing assault by Masaki Tokishige, Tanetoshi resisted to the end and, at the Second Battle of Kōnodai, his loyalty was questioned by the Hōjō after which he betrayed them in favor of the Satomi clan. In the latter part of the Eiroku era (1558 to 1570), Tanetoshi transferred headship of the clan to his son, Sakai Masatoki, and adopted the monk’s name of Gentetsu.