Ryōō 了翁 (active mid-Kamakura period, traditional lifedates uncertain — conventionally placed in the 13th century) was a Tendai esoteric (Taimitsu 台密) master whose teaching on the Garbhadhātu and Vajradhātu kanjō (consecration) procedure was recorded by his disciples in the single-fascicle KR6t0115 Guàndǐng sī jiànwén 灌頂私見聞 (“Private kanjō Notes from What I Have Heard and Seen”). The work opens explicitly with the words “Ryōō’s teaching” 了翁御談.
Ryōō is sparsely documented in the standard biographical sources, but his kanjō teaching as preserved in the Guàndǐng sī jiànwén shows that he had detailed command of the Tendai-esoteric Garbhadhātu kanjō procedure — covering the etymological-doctrinal interpretation of kanjō, the seven-day altar-construction sequence, the eight-color banner arrangement, and the twelve-deva invitation. He is to be distinguished from the early Edo-era Ōbaku Zen master 了翁道覺 (Ryōō Dōkaku, 1630–1707).