Trichodermaerin (1), a novel diterpenoid lactone, together with the known compound, harziandione (2) were isolated from the culture broth of the fungus Trichoderma erinaceum associated with the sea star Acanthaster planci. Their structures were determined by analysis of the NMR and MS data. 1 was the Baeyer-Villiger monooxygenase catalyzed oxidation product of 2. Compound 2 did not show cytotoxic activities against various cancer cell lines.