Three new bioactive marine diterpenoids, stolonidiol (1), stolonidiol monoacetae (2), and claenone (3), were isolated from Japanese soft coral of the genus Clavularia. The structures of these diterpenoids were elucidated on the basis of spectroscopic data including two-dimensional nuclear magnetic resonance, chemical reactions, and/or X-ray crystallographic analysis. Compounds 1 and 2 showed a strong cytotoxic activity against P388 leukemia cells, and 3 was inhibitory in the fertilized sea urchin egg assay.