Lifespan: Tenbun 3 (1543) to Bunroku 2 (1593)
Lord: Chōsokabe Kunichika → Chōsokabe Motochika
Father: Chōsokabe Kunichika
Adoptive Father: Kōsokabe Chikahide
Siblings: Motochika, Kira Chikasada, Kōsokabe Chikayasu, Shima Chikamasu, sister (wife of Motoyama Shigetoki), sister (wife of Ike Yorikazu), sister (wife of Hakawa Kiyomune)
Wife: [Formal] Daughter of Kōsokabe Hidemichi
Children: Chikauji, Sadachika
Kōsokabe Chikayasu served as a bushō during the Sengoku and Azuchi-Momoyama periods. He was a retainer of the Chōsokabe clan of Tosa Province in Shikoku.
In 1543, Chikayasu was born as the third son of Chōsokabe Kunichika, the twentieth head of the Chōsokabe clan. His childhood name was Yashichirō.
In 1558, upon orders of his father, he was adopted by Kōsokabe Chikahide. In 1569, after Aki Kunitora, a bushō and kokujin, or provincial landowner, in Tosa, was eliminated, Chikayasu became the lord of Aki Castle.
Later, he followed his older brother, Chōsokabe Motochika (a sengoku daimyō and twenty-first head of the Chōsokabe clan), to engage in assorted battles. Chikayasu supported efforts by Motochika to pacify Awa Province, conducting battles from a base at Kaifu Castle in the Kaifu District of Awa. In 1579, he captured Tomioka Castle from Shingai Sanetsuna. In addition to his military endeavors, Chikayasu excelled in diplomacy. In 1575, Oda Nobunaga served in a ceremonial role at the coming-of-age ceremony for Chōsokabe Nobuchika to place on Nobuchika his black-lacquered headgear known as eboshi. In 1580, Chikayasu crossed the Seto Inland Sea and met Nobunaga at Azuchi Castle in Ōmi Province. Chikayasu sought reconciliation with Miyoshi Yasunaga and the Miyoshi clan.
In the sixth month of 1582, after the demise of Nobunaga in a coup d’état known as the Honnō Temple Incident, Chikayasu contributed to the defeat of Sogō Masayasu at the Battle of Nakatomigawa, and, in 1583, attacked Kizu Castle in Awa, supporting Motochika’s aim to unify Shikoku under his control.
Following the death of Nobunaga, Chikayasu cooperated with Shibata Katsuie and Tokugawa Ieyasu to facilitate progress in efforts to unify Shikoku. Letters addressed to him demonstrate that Chikayasu served as the key point of contact in the Chōsokabe clan for dealings with assorted daimyō families including the Oda, the Tokugawa, and others. In 1585, during the Invasion of Shikoku by Toyotomi Hideyoshi, Chikayasu defended Ushiki Castle in Awa. After Kizu Castle fell to the Toyotomi army, Chikayasu abandoned Ushiki and returned to Tosa Province.
In 1592, while heading out for the Bunroku Campaign on the Korean Peninsula, his eldest son, Chōsokabe Chikauji, suddenly died. Meanwhile, Chikayasu himself died in Nagato Province prior to sailing to Korea. He was fifty-one years old.
Nakayamada Yasuyoshi (the son of Kōsokabe Hidemichi and younger brother of Kōsokabe Chikahide) was given consideration as a successor but, in the end, Chikayasu was succeeded by his second son, Chōsokabe Sadachika while Yasuyoshi served as his guardian. Similar to the loss of Kira Chikasada, the death of Chikayasu was a significant blow to the Chōsokabe clan.
Chikayasu, as an alter ego of Motochika, engaged in battles across Shikoku, loyally serving his older brother. With the death of Chikayasu, no one remained to support as well as to admonish Motochika, so, thereafter, Motochika committed a series of reckless acts, leading to the decline of the Chōsokabe clan. After the fall of Motochika, the clan was removed from its position of influence in Shikoku.