A new trinor-guaiane sesquiterpene has been isolated from an Indonesian soft coral Anthelia sp. The planar structure, possessing an octahydroazulene skeleton, and relative stereochemistry were established by analyzing 1D and 2D NMR data, including NOE experiments. Its absolute stereochemistry was elucidated to be 1S, 4S, 7R, and 10R by comparing observed and calculated optical rotation values. The new compound showed weak cytotoxicity against NBT-T2 cells.