The asymmetric unit of the title compound, C₈H₉NO₂·H₂O consists of an isolated 4-(ammonio-meth-yl)benzoate zwitterion derived from 4-amino-methyl-benzoic acid through the migration of the acidic proton, together with a water molecule of crystallization that is disordered over three sites with occupancy ratios (0.50:0.35:0.15). In the crystal structure, N–H⋯O hydrogen bonds together with π–π stacking of the benzene rings [centroid-centroid distance = 3.8602 (18) Å] result in a strongly linked, compact three-dimensional structure.