A new lupin alkaloid, (+)-13β-hydroxymamanine (1), was isolated from the fresh stems of Maackia amurensis Rupr. et Maxim. var. buergeri C. K. Schn. (Leguminosae) via repeated silica gel chromatography (yield: 0.007% of fresh weight). Its molecular formula was established as C₁₅H₂₂N₂O₃ by high resolution electron impact mass spectrometry (HREIMS). IR and UV spectra indicated the presence of a 2-pyridone moiety. The structure, including the 13β (axial) hydroxy configuration, was elucidated through ¹³C- and ¹H-NMR spectra (using selective proton decoupling and homonuclear decoupling experiments) and confirmed by chemical transformation to its 10,13-diacetate (2) and N-oxide (3). The new alkaloid's structure corresponds to an oxidative product derived from the N₁-C₁₀ cleavage of (-)-baptifoline (8) coexisting in the plant, suggesting biosynthetic relationships between bridged quinolizidine alkaloids and unbridged bases.