He was born a son of Ihara Masamori, of the Ihara family, senior retainers of the Imagawa of Suruga. His mother is said to have been a daughter of the Okitsu, also Imagawa vassals. Though a warrior's son, he entered the Buddhist clergy as a child and followed the path of learning as a monk.
He had been studying under Joan Ryuso at Kenninji in Kyoto, but is said to have been called back to Suruga by Imagawa Ujichika to tutor his fifth son Hogikumaru, the future Yoshimoto. The two entered Zentokuin and later went up to Kyoto to study Zen at Kenninji and Myoshinji.
The clan head Imagawa Ujiteru and his brother died suddenly on the same day, and Yoshimoto fought his half-brother Genko Etan for the succession (the Hanakura Rebellion). Sessai had Yoshimoto return to lay life, rallied his side with Ujichika's widow Jukeini and others, led it to victory and made Yoshimoto clan head.
Yoshimoto took a daughter of Takeda Nobutora of Kai as his wife and allied with the Takeda; Sessai is thought to have handled the talks. Angered, Hojo Ujitsuna of Sagami invaded the Kato region of eastern Suruga, starting the conflict between the Imagawa and Hojo known as the Kato War.
By this time Sessai was using the name Taigen Sufu as a Zen monk of the Myoshinji school. While continuing his training and studies as a monk, he was deeply involved in the government, diplomacy and even military command of the Imagawa, and had become Yoshimoto's foremost adviser.
With Takeda Harunobu as mediator, Sessai arranged peace with Hojo Ujiyasu and won back for the Imagawa the Kato lands the Hojo had taken. Around the same time he founded Rinzaiji in Sunpu to pray for Yoshimoto's brother Ujiteru, and later served as its abbot.
The Imagawa extended their power west into Mikawa, and Sessai led an army that took Tahara Castle there. The Matsudaira of Mikawa relied on the Imagawa, and it is said that Takechiyo, the future Tokugawa Ieyasu, was seized by the Oda of Owari on his way to Sunpu as a hostage at this time.
The Imagawa army under Sessai fought Oda Nobuhide's forces, which had invaded Mikawa, at Azukizaka, and the Imagawa won. The victory strengthened the Imagawa position in Mikawa, and Oda power there gradually receded.
After Matsudaira Hirotada died, Sessai took Anjo Castle in Mikawa and captured its commander Oda Nobuhiro, a son of Nobuhide. He then arranged an exchange of Nobuhiro for Takechiyo, who was held by the Oda, and Takechiyo was placed with the Imagawa in Sunpu.
Sessai was installed as abbot of Myoshinji in Kyoto, rising high as a Zen monk while also taking the field as a commander. There is a story that Takechiyo was taught by Sessai in Sunpu, but accounts differ and it is not certain.
Yoshimoto issued a supplement adding new articles to the Imagawa Kana Mokuroku, the domain law made by his father Ujichika; Sessai is thought to have helped draft it. It declared that the Imagawa governed by their own power rather than relying on the authority of the Muromachi shogunate.
The Imagawa, Takeda and Hojo formed the Kai-Sagami-Suruga Tripartite Alliance, sealed by exchanging daughters in marriage, and Sessai is said to have worked hard to bring it about. The story that the three lords met at Zentokuji is thought to be a later invention.
That year he died of illness at Chokeiji in Suruga, at sixty by traditional count. The Imagawa kept their power for a while after losing him, but five years later Yoshimoto was defeated by Oda Nobunaga at the Battle of Okehazama and died, and the clan declined rapidly.
Hōjō UjiyasuOda NobunagaTokugawa IeyasuImagawa YoshimotoHojo Ujitsuna
Hōjō Sōun (Ise Sōzui)Amago TsunehisaHojo UjitsunaSaitō DōsanMōri MotonariFrancis XavierŌuchi YoshitakaMatsunaga Hisahide
People of the Sengoku/Azuchi-Momoyama period ・ People by period
Based on facts found in textbooks and biographies. Uncertain points are marked as such.