An efficient synthesis of nucleoside 5'-monothiophosphates under mild reaction conditions using commercially available thiophosphoryl chloride was achieved with a cinchona alkaloid catalyst. A detailed mechanistic study of the reaction was undertaken, employing a combination of reaction kinetics, NMR spectroscopy, and computational modeling, to better understand the observed reactivity. Taken collectively, the results support an unprecedented mechanism for this class of organocatalyst.