Spectroscopic methods and X-ray diffraction analysis were used to elucidate the structure of stephadiamine, a novel pentacyclic C-norhasubanan alkaloid isolated from Stephania japonica—a Chinese medicinal herb employed as an anti-diarrheal, anti-anetus, anti-febrile, tonic, diuretic, and remedy for podagra and cholera. The ethanolic extract of S. japonica collected in Taiwan was previously reported to yield alkaloids like metaphanine; stephadiamine (1) was isolated with difficulty (4×10⁻⁶ yield) via repeated alumina column and preparative thin-layer chromatography from the mother liquor after metaphanine removal. Characterized as C₁₉H₂₄N₂O₄ (by high-resolution mass spectrometry and elemental analysis), it features IR bands for NH₂ (3375 cm⁻¹) and δ-lactone (1720 cm⁻¹), along with ¹H-NMR signals for N-CH₃, two OCH₃ groups, and specific protons (e.g., C-10-H). Derivatization (N-acetyl-stephadiamine, hydroxy ester) supported structural features. X-ray diffraction of (1) (monoclinic, space group P2₁, R=0.037) and N-p-bromobenzoylstephadiamine (5)—used to resolve absolute configuration—confirmed a hasubanan-like pentacyclic skeleton. Stephadiamine is the first natural lactonic C-norhasubanan alkaloid and an N₂-base, distinct from known natural hasubanan bases (which are N₁-bases with one nitrogen atom).