Sporulaminals A (1) and B (2), a pair of unusual epimeric spiroaminal derivatives, bearing a 6/4/5/5 tetracyclic ring system derived from bergamotane sesquiterpenoid, were isolated from a marine-derived fungus Paraconiothyrium sporulosum YK-03. Their structures including the absolute configurations were elucidated by extensive NMR experiments, crystal X-ray diffraction and computational electronic circular dichroism (ECD) method. Furthermore, the epimerization induced by pH, temperature and H₂O was revealed, together with the hypothetical biosynthetic pathway.