A new cembrane-type diterpenoid, sinulaflexiolide P (1), along with three known derivatives: sinulaflexiolide H (2), 11-epi-sinulariolide acetate (3), and (1S*,3S*,4S*,7E,11E)-3,4-epoxy-13-oxo-3,7,11,15-cembratriene (4), was isolated from a population of Bornean soft coral Sinularia flexibilis. The structures of these metabolites were elucidated based on spectroscopic data including NMR and HR-ESI-MS. In addition, these compounds were tested against six strains of marine fungi.