Aspermeroterpene A (1) with an unprecedented and highly congested 5/3/6/6/6/5 hexacyclic skeleton, together with two precursors aspermeroterpenes B (2) and C (3), were isolated from the marine-derived fungus Aspergillus terreus GZU-31-1. Their structures were elucidated by quantum chemical calculations, X-ray diffraction, and spectroscopic methods. The biogenetic pathway for 1-3 is proposed. Aspermeroterpenes A-C (1-3) showed significant inhibitory activities against lipopolysaccharide (LPS)-induced nitric oxide (NO) production in RAW 264.7 cells compared to positive control.