The structure of sargatriol, a new phenolic compound containing an oxygenated isoprenoid component isolated from a brown alga Sargassum tortile, was elucidated to be 3 on the basis of spectral and chemical evidence. The absolute stereochemistry was established from the circular dichroism (CD) of sargatriol (3) and the shift reagent-induced CD of the methyl ether (4).