A new sesquiterpenoid, diaporol A (1), possessing a unique tricyclic lactone framework, eight new drimane sesquiterpenoids, diaporols B-I (2-9), and the known compounds 10 and 11 were isolated from a culture of the mangrove-derived endophyte Diaporthe sp. The absolute configurations of 1-5 were determined by low-temperature (100 K) single-crystal X-ray diffraction with Cu Kα radiation. The compounds were evaluated for cytotoxic activity; however, no compound showed significant cytotoxicity against the tested cell lines at a concentration of 20 μM.