Saeki Koresada served as a bushō during the Azuchi-Momoyama and early Edo periods.  Koresada was the fourteenth head of the Bungo-Saeki lcan and the lord of Togamure Castle in the Amabe District of Bungo Province.  Later, he was a senior retainer of the Tsu domain in the Anō District of Ise Province.

In 1569, Koresada was born as the second son of Saeki Koresane, the thirteenth head of the Saeki clan of Bungo.

In 1578, his father, Koresane, and grandfather, Saeki Korenori, were killed in action at the Battle of Mimikawa, upon which Koresada inherited the family.  In 1586, during the Hōsatsu War, local commanders surrendered one after another to the Shimazu clan.  Koresada assigned a guest commander named Yamada Munemasa to engage in fierce resistance to the Shimazu.  On 11/4 of Tenshō 14 (1586), when Shimazu Yoshihisa attacked Koresada at his base at Togamure Castle, Koresada crushed the enemy forces in an event known as the Battle of Katata.  On 12/4, a retainer of the Saeki named Takahata Shinemon toppled Hoshiko Castle and captured the wife and child of Shibata Shōan who betrayed the Ōtomo in favor of the Shimazu.  As a result, Shōan was shaken and sought to return to the service of the Ōtomo, but, upon discovery by the Shimazu, he was killed.

On 12/18, Koresada led an attack against a logistics division of the Shimazu army led by Todaka Shōgen at Inbidani, eviscerating them.  In the second month of 1587, Koresada recaptured Asahidake Castle defended by Tsuchimochi Chikanobu.  When, after requests from Ōtomo Yoshishige (Sōrin), Toyotomi Hideyoshi launched the Pacification of Kyūshū, on 3/17 of Tenshō 15 (1587), Koresada roundly defeated an army led by Shimazu Yoshihiro and Shimazu Iehisa (siblings) during their retreat from Funai at the Azusa Pass on the border of Hyūga and Bungo provinces in Kyūshū.  After the arrival in Bungo of the general for the road to Hyūga, Toyotomi Hidenaga, Koresada converged with his forces, serving as a guide to enter Hyūga and participating in an attack on Taka Castle.  After the Pacification of Kyūshū, Hideyoshi praised Koresada for his valiant efforts on the battlefield and awarded him with a letter of commendation.

In 1592, for the Bunroku Campaign on the Korean Peninsula, Koresada participated as a member of the Ōtomo army but owing to the removal of the Ōtomo clan from their position, Koresada also lost his base at Togamure Castle.  Koresada relied upon Toyotomi Hideyasu (the successor to Toyotomi Hidenaga) and, after the demise of Hideyasu, was invited as a guest commander to served Tōdō Takatora with a rice stipend equivalent to the labor of 500 men.  In 1595, Koresada followed Takatora upon his transfer to Uwajima in Iyo, and, the next year, was granted a fief of 2,000 koku.  Later, Koresada served as the chamberlain of Kō Castle in lieu of Tōdō Yoshikatsu.  During the Keichō Campaign in 1597, Koresada guarded Itashima Castle in Uwajima and, initially, only his retainers deployed for the campaign.  Later, Koresada also deployed and served valorously in a sea battle.

In 1600, during the Battle of Sekigahara, Koresada stayed behind to defend Uwajima Castle in Iyo Province while only his retainers deployed for the main battle.  From the next year, Koresada engaged in construction activities led by Takatora, establishing the town of Saeki below Tsu Castle in Ise.  In the tenth month of 1614, at the Winter Campaign of the Siege of Ōsaka, he led a unit comprised of ten local fighters and forty soldiers.  In the beginning, Koresada served as a general of hatamoto, but, from 11/26 of Keichō 19 (1614), he joined the right wing of the vanguard forces under Tōdō Takanori.

The next year, at the Summer Campaign of the Siege of Ōsaka, Koresada was ordered to lead a reserve corps, but in a battle occurring on 5/6 of Keichō 20 (1615), the vanguard was decimated so, on 5/7, Koresada and Tōdō Takayoshi joined the left wing of the vanguard.  After the war, he received an increase to his fief to 4,500 koku.

Koresada died on 6/9 of Genna 4 (1618).  He was fifty years old.  His eldest son, Saeki Koreshige, inherited the headship of the family which served as retainers of the Tōdō family until the Meiji period.