Tani Tadazumi served as a bushō during the Sengoku and Azuchi-Momoyama periods. He was a retainer of the Chōsokabe clan of Tosa Province. His younger brother, Hiyū, was a monk in the Shingon sect.
In 1534, Tadazumi was born. Originally, he served as the chief priest at the Tosa Shrine which was ranked as an ichinomiya, or shrine of the highest status in the area. Later, he was identified by Chōsokabe Motochika, the sengoku daimyō of Tosa, and became a retainer of the clan. He was primarily active in diplomatic affairs.
In the eleventh month of 1584, after the Battle of Komaki-Nagakute, Hashiba Hideyoshi settled with Tokugawa Ieyasu and Oda Nobukatsu. In the Invasion of Kishū in the fourth month of 1585, Hideyoshi garnered control of Kii Province and placed his younger brother, Hashiba Hidenaga, at Wakayama Castle. He then expressed an intention to conquer Shikoku. Many in the band of retainers of the Chōsokabe feared a confrontation against Hideyoshi so they convinced Motochika to reach a peace deal with him. Motochika dispatched Tadazumi to request recognition from Hideyoshi to the rights of the Chōsokabe to the entire territory of Shikoku. Hideyoshi promised to return Sanuki and Awa provinces and to recognize the rights of the Chōsokabe to Iyo and Tosa provinces. Before long, however, friction arose between the two sides and Motochika was the subject of Hideyoshi’s campaign of subjugation.
In the sixth month of 1585, Hideyoshi, with Hidenaga serving as the commander-in-chief, initiated the Invasion of Shikoku. At this time, Tadazumi, along with Emura Chikatoshi fought valiantly in defense of Ichinomiya Castle on the front lines in Awa Province. For the defense of the castle, Motochika mobilized forces including among the jizamurai, or local fighters (who worked as peasants during times of peace), from the area. In all, approximately 9,000 soldiers amassed to defend the castle. This garrison was opposed by over 50,000 troops in the Hashiba army led by Hachisuka Masakatsu, Tōdō Takatora, Mashita Nagamori, Sengoku Hidehisa, Toda Katsutoshi, and Hitosuyanagi Naosue. After laying siege to the castle from the north, east, and west, the besieging forces fired arquebuses in unison and made multiple attempts to storm the castle. Nevertheless, the defenders maintained a high level of morale so it turned into a prolonged battle. Confronted with stiff resistance by the defenders, Hidenaga cut-off their water supplies and had his men dig tunnels to destabilize the foundations of the castle. This caused major concern among those in the castle so Chikatoshi and Tadazumi decided to vacate.
After the fall of Ichinomiya Castle, Tadazumi returned to Hakuchi Castle in Awa and, based on the superiority in numbers as well as quality of weapons of the Hashiba army, advised his lord, Motochika, to surrender. Senior retainers opposed the prospect of surrender, but the deliberations ultimately leaned in favor of surrender. Embarrassed at surrendering without even a single fight, Motochika announced a strategy to resist at Kaifu, asserting that even if the invading forces reach their home province, they should resist to the end. He then ridiculed Tadazumi for advocating surrender, ordering him to commit seppuku. These words caused all present to commit to fight, but Tadazumi did not yield and persuaded the senior retainers to jointly sign a document requesting Motochika to reconsider. In the end, Motochika agreed and, in a document dated 7/25, accepted the terms for a cessation of hostilities from Hashiba Hidenaga and surrendered.
After the Invasion of Shikoku, in 1586, Tadazumi served in the Subjugation of Kyūshū by the Toyotomi army. He then participated in the Battle of Hetsugigawa in Bungo Province. In the course of the battle, Tadazumi served as a messenger for Chōsokabe Motochika to receive the return from Niiro Tadamoto of the remains of Motochika’s eldest son, Chōsokabe Nobuchika, who had been killed by Yagi Masanobu, a retainer of the Shimazu. This mission occurred upon orders of Motochika who, after a crushing defeat at the Hetsugi River, narrowly escaped to Hiburi Island in the waters off of Iyo Province. Tadamoto treated Tadazumi cordially and wept while apologizing for Nobuchika’s death in battle. Tadamoto then had Nobuchika’s remains cremated and dispatched a monk to accompany Tadazumi on the trip to return the remains to the base of the Chōsokabe at Okō Castle in Tosa.
After returning from Kyūshū, he became the chamberlain of Nakamura Castle in the Hata District of Tosa. He managed civil administration for the area including by having prisoners raise pine trees on Irino Beach in the western portion of the province.
In 1600, Tadazumi died at Tosa-Nakamura Castle. He was sixty-seven years old.