A new general de novo synthesis of pharmaceutically important N-(hetero)aryl piperidines is reported. This protocol uses a robustly diastereoselective reductive amination/aza-Michael reaction sequence to achieve rapid construction of complex polysubstituted ring systems starting from widely available heterocyclic amine nucleophiles and carbonyl electrophiles. Notably, the diastereoselectivity of this process is enhanced by the presence of water, and DFT calculations support a stereochemical model involving a facially selective protonation of a water-coordinated enol intermediate.