Ternatusine A (1), a novel alkaloid with an unprecedented epoxyoxepino[4,5-c] pyrrole ring, was isolated from the roots of Ranunculus ternatus Thunb. Its unusual structure, including its absolute stereochemistry, was determined using UV, IR, HRESIMS, and 1D and 2D NMR data and through comparison of the experimental and calculated electronic circular dichroism (ECD) spectra. A possible biosynthetic pathway for ternatusine A was postulated.