Lifespan: Eishō 8 (1511) to 12/13 of Tenshō 10 (1583)
Other Names: Jinshirō, Heiemon
Lord: Matsudaira Kiyoyasu → Matsudaira Hirotada → Tokugawa Ieyasu
Father: Utsu Tadashige
Siblings: Tadahira, Tadatoshi, Tadatsugu, Tadakazu, Tadahisa, Tadayuki, sister (wife of Katō Kageshige), sister (wife of Sugiura Masatsugu)
Children: Tadayo, Tadasuke, Tadakane, 忠核, Tadatame, Tadanaga, Tadanori, Tadamoto, Kuheiji, daughter (wife of Ōkawachi Masatsuna)
Ōkubo Tadakazu served as a bushō during the Sengoku and Azuchi-Momoyama periods. A retainer of the Tokugawa, Tadakazu is counted among the bushō who served valorously during the Siege of Kanie Castle known as the Seven Spears of Kanie.
In 1511, Tadakazu was born as the son of Utsu Tadashige, a retainer of the Matsudaira clan of Mikawa Province and the founder of the Ōkubo clan.
Tadakazu was a veteran who served three generations of lords beginning with Matsudaira Kiyoyasu, the grandfather of Tokugawa Ieyasu.
In 1537, when Matsudaira Hirotada fled to Ise Province, Tadakazu and his older brother, Ōkubo Tadatoshi, ushered him in to Okazaki Castle. In 1542, during a rebellion by Matsudaira Nobutaka, when Tadakazu’s younger brother, Ōkubo Tadahisa, supported Nobutaka, Tadakazu and his older brother, Tadatoshi, convinced Tadahisa to return to the service of Hirotada.
In 1555, during a Siege of Kanie Castle, Tadakazu served valorously and, in later eras, was counted among a group of bushō known as the Seven Spears of Kanie.
In 1563, Tadakazu defended Kamiwada Castle during the Mikawa ikkō-ikki.