Lifespan: Eiroku 7 (1564) to 10/20 of Kanei 2 (1625)
Rank: bushō, daimyō
Title: Junior Fifth Rank (Lower) and Governor of Nagato
Lord: Oda Nobunaga → Toyotomi Hideyoshi → Toyotomi Hideyori → Tokugawa Ieyasu → Tokugawa Hidetada
Domain: Head of Kameyama in Ise, Tara in Mino, and Kurosaka in Hōki
Father: Seki Morinobu
Mother: Daughter of Gamō Sadahide
Siblings: Kazumasa, Moriyoshi, Moritada, Kazutoshi, Ujitoshi
Wife: Daughter of Gamō Katahide
Seki Kazumasa served as a bushō and daimyō during the Azuchi-Momoyama and early Edo periods. He was the lord of the Kameyama domain in Ise Province, the Tara domain of Mino Province, and the Kurosaka domain of Hōki Province.
Kazumasa was born as the second son of Seki Morinobu. Along with his father, Kazumasa served Oda Nobunaga and Toyotomi Hideyoshi. In 1584, he participated in the Battle of Komaki-Nagakute.
Thereafter, he was assigned headship of the clan by his father, and, upon orders of Hideyoshi, became the daimyō of security for Gamō Ujisato. He participated in the Pacification of Kyūshū and the Conquest of Odawara. In 1591, he achieved meritorious results in the subjugation of Kudo Masazane so, after Ujisato received a large territory in Aizu, he was awarded a fief of 50,000 koku and Shirakawa Castle.
In 1596, Kazumasa was invested with the title of Junior Fifth Rank (Lower) and the Governor of Nagato. He was also conferred the Toyotomi surname.
In 1598, Kazumasa was reassigned to Iiyama in Mino with a fief of 30,000 koku. He was further appointed as the official in charge of the four districts of Kawanakajima in northern Shinano Province (comprised of the Minochi, Taka, Sarashina, and Hanishina districts). In 1600, he was transferred to Tara in Mino Province.
In the ninth month of 1560, at the Battle of Sekigahara, initially, he defended Inuyama Castle in Owari Province in support of the Western Army. Later, he switched sides to the Eastern Army. At the main battle, he contributed in a division led by Ii Naomasa. After the war, his contributions were recognized by permitting him to be reinstated at the ancestral home of the Seki clan at Kameyama in Ise Province.
In 1611, two years after the death of Nakamura Kazutada at the age of twenty, the Yonago domain in Hōki was extinguished owing to the absence of a natural heir and rejection of the appointment of an adopted heir. Following this development, Kazumasa was then transferred to Kurosaka in Hōki with a fief of 50,000 koku. In 1614, at the Winter Campaign of Ōsaka, Kazumasa led an attack on Kyōbashiguchi, and, the following year, at the Summer Campaign of Ōsaka, attacked Kyōbashiguchi and claimed fifty-two heads.
In 1618, owing to an internal dispute, he was removed from his position. However, his adopted son, Seki Ujimori (the eldest son of his younger brother, Seki Moriyoshi), was granted 5,000 koku in Gamō in Ōmi Province and permitted to continue to use the family name.