Alopecurin A, an alkaloid with an unprecedented skeleton, was isolated from the seeds of Sophora alopecuroides L. The absolute configuration and structure of this compound was identified as (3S,12R)-3-hydroxy-1,7-diazatricyclo[10.4.0.13,7]heptadecane-11,16,17-trione (=(7S,15aR)-decahydro-7-hydroxy-6H-7,11-methano-4H-pyrido[1,2-a][1,7]diazacyclododecine-4,15,16(12H)-trione). The structure and absolute configuration was elucidated by spectroscopic methods, mainly HR-ESI-TOF-MS, IR, 1D-NMR (1H- and 13C-NMR), 2D-NMR (COSY, NOESY, HSQC, HMBC), and particularly X-ray crystal-diffraction and CD spectral analysis.