Ōkubo Tadatsune served as a bushō and daimyō during the Azuchi-Momoyama and early Edo periods.  Tadatsune was the first lord of the Kisai domain in Musashi Province.

In 1580, Tadatsune was born as the lineal heir of Ōkubo Tadachika, a retainer of the Tokugawa clan and the lord of the Odawara domain in Sagami Province.

From his youth, Tadatsune was an exceptional individual liked by Tokugawa Ieyasu and Tokugawa Hidetada (father and son serving as the first and second shōgun of the Edo bakufu respectively).  During his coming-of-age ceremony, held in the presence of Hidetada together with the children of hereditary retainers in Mikawa, he received the character “tada” from the name of Hidetada.  He was also awarded a fief of 20,000 koku in Kisai in Musashi (separate from the landholdings of his father) reflecting the expectations for Tadatsune to exercise his talents on behalf of the Edo bakufu in the future.  Tadatsune himself was a compassionate and sincere individual, deeply respected by others so that his father also held high hopes for his future.

On 10/10 of Keichō 16 (1611), Tadatsune died at the age of thirty-two.  Although one account notes that he died of illness, it states “Despite Tadatsune’s youth, the powerful Sado-no-kami (Honda Masanobu) came after him.  Masanobu was very jealous of Tadatsune, wishing for his demise.  These feelings of ill will extended even to Tadatsune’s subordinates.”  During this period, Honda Masanobu and Honda Masazumi (father and son) were vying with Tadachika for power so this could be interpreted to mean that Tadatsune was a target of assassination by them instead.

From the preceding spring, he fell ill.  The bloodline of Masanobu’s branch of the Honda family came to an end.  In a compilation of the Tokugawa clan from the late-Edo period when the Ōkubo family continued to survive, it is not unexpected that the contents were flattering to the Ōkubo family.  In one account, Tadatsune was the most prominent associate of the shōgun at the time and many persons owed him a debt of gratitude so those who made a condolence call to him in Odawara without the permission of their lord were subject to house arrest.

Distraught at the sudden loss of his eldest son, Tadatsune’s father, Tadachika, sequestered himself in his home and neglected his political duties.  In 1614, Tadachika was removed from his position.  Another one of his sons, Ōkubo Tadamoto, was pardoned but confined to Kisai Castle in Musashi.  Meanwhile, Morikawa Shigetoshi and Kusakabe Masafuyu were placed under house arrest for making condolence calls to Tadachika without permission of the bakufu.