Nine cembrane-type diterpenoids, manaarenolides A-I (1-9), along with two known cembranolides, 10 and 11, have been isolated from the ethyl acetate extract of the Taiwanese soft coral Sinularia manaarensis. Among these metabolites, diterpenes (1, 2, 5, and 6) were discovered for the first time as the hydroperoxycembranolides possessing a delta-lactone ring. The 13-epimeric metabolites 7 and 8 have been shown to exhibit moderate cytotoxic activity against Hepa59T/VGH, KB, Hela, and Med cancer cell lines. The biosynthetic relationship between these new metabolites and 10 and 11 is also discussed.