Antroalbocin A (1), a sesquiterpenoid possessing a bridged tricyclic system, was isolated from cultures of the higher fungus Antrodiella albocinnamomea (Basidiomycota). The structure with the absolute configuration was determined by extensive spectroscopic methods and single-crystal X-ray diffraction. A plausible biosynthetic pathway for 1 was proposed. Compound 1 was found to inhibit Staphylococcus aureus with an MIC value of 169 μM.