Molecular-level understanding of the early stage of crystallization remains a significant challenge. For a wide range of fundamental research and applications, it is of great importance to understand the mechanism of crystal polymorph selections in different solvents or in the presence of additives. We studied glycine crystallization in aqueous solution with or without the addition of sodium chloride (NaCl), one crystal nucleation at a time, using the recently developed single-crystal nucleation spectroscopy (SCNS). It has been reported that glycine forms γ-glycine when salt is added to aqueous solution, and the mechanism has been considered as a classical nucleation where γ-glycine forms directly as a crystal nucleus. Our study shows that metastable polymorph β-glycine forms first in aqueous solution both with or without NaCl through a nonclassical nucleation pathway. NaCl can stabilize the metastable β-glycine over several hours and also prevent it from converting to α-glycine. Eventually γ-glycine nucleates on the surface of β-glycine and then grows while dissolving the mother β-glycine. The crystal habit of β-glycine suggests that the stabilization by NaCl occurs at its polar face. SCNS provides crucial information to accelerate the investigation of the early stage of crystallization toward the rational control of polymorphisms.