Sosei (dates of birth and death unknown, died in 910?) was a poet and a monk who lived during the early and middle Heian period. He was one of the Thirty-six Immortal Poets. His father was Sojo Henjo (YOSHIMINE no Munesada). There are various opinions about his secular name. According to one theory, it was YOSHIMINE no Harutoshi. The origin of the Yoshimine clan goes back to YOSHIMINE no Yasuyo, who was the son of Emperor Kanmu, demoted from nobility to subject, therefore Sosei was a great-grandchild of Emperor Kanmu. He was born when his father Henjo was a layman, and seems to have been made to become a priest together with his older brother, Yusei.
Sosei took a lively part in composing waka poems as a monk who was close to the Imperial court together with his father, Henjo. At first he served the Imperial court and was advanced to tenjobito (a high-ranking courtier allowed into the Imperial Palace), but retired to the priesthood when he was still young. When Imperial Prince Tsuneyasu, a son of Emperor Ninmyo, became a priest and fixed his residence at the Urin-in Temple, Henjo and Sosei were allowed to visit the prince. After the death of the prince, Henjo was entrusted with management of the Urin-in Temple, and even after Henjo passed away, Sosei lived there and the temple was known as the place where the gatherings of waka poetry and Chinese poetry were held. Later, he moved to Ryoin-in Temple in Yamato. He was often invited to the uta-awase (poetry contest) held by Emperor Uda and composed poems.
His poems were collected in the Imperial Anthologies such as Kokin shu (abbreviation for Kokin Wakashu - A Collection of Ancient and Modern Japanese Poetry), and one of his poems was also chosen for FUJIWARA no Teika's Ogura Hyakunin Isshu (the Ogura Anthology of One Hundred Tanka-poems by One Hundred Poets). His personal collection of poetry is "Sosei shu" (A Collection of Poems by Sosei) (the poems were selected by others).