The conformational and structural properties of the bicyclic quinolizidine alkaloid (-)-lupinine have been investigated in a supersonic jet expansion using microwave spectroscopy. The rotational spectrum is consistent with a single dominant trans conformation within a double-chair skeleton, which is stabilized by more than 10.4 kJ mol(-1) with respect to other conformations. In the isolated conditions of the jet the hydroxy methyl side-chain of the molecule locks-in to form an intramolecular O-H•••N hydrogen bond to the electron lone-pair at the nitrogen atom. Accurate rotational constants, centrifugal distortion corrections and (14N) nuclear quadrupole coupling parameters are reported and compared to ab initio (MP2) and DFT (M06-2X) calculations. The stability of lupinine is further compared computationally with epilupinine and decaline in order to gauge the influence of intramolecular hydrogen bonding, absent in these molecules.