Saien 最圓 (1048–1102, traditional dates) was a mid-Heian Tendai esoteric (Taimitsu 台密) master in the Kawa-ryū 川流 lineage on Mt. Hiei, in the line of descent from Kōkei 皇慶 (皇慶). He is recorded as one of the Tendai ajari who developed the structural-analytical (shōki 生起) genre of liturgical exposition, applying the framework refined by Kakuchō (覺超) to the Vajra-realm 金剛界 (Vajradhātu) liturgy.
His single surviving work in the Kanripo corpus is the KR6t0105 Jīngāngjiè cìdì shēngqǐ 金剛界次第生起, a one-fascicle structural exposition of the Vajradhātu liturgical sequence.
He should be distinguished from the later Tendai monk of the same name and from the Shingon-school 最圓 of the Ono lineage.