Seven new marine diterpenoids having a cembrane skeleton were isolated from the Okinawan soft coral Clavularia koellikeri. Their structures were determined based on the results of spectroscopic analysis and chemical conversions. Compound 1 showed cytotoxic activity against human colorectal adenocarcinoma cells (DLD-1, IC(50) 4.2 g/mL) and strong growth inhibition against human T lymphocytic leukemia cells (MOLT-4, IC(50) 0.9 g/mL).