A facile method to synthesize WO₃·H₂O square nanoplates via a mild aging (50°C) of ion-exchanged precursor was developed. The ion-exchanged precursor was prepared by passing the sodium tungstate solution (Na₂WO₄) through a protonated cation-exchange resin, and used as impurity-free acidified solution (H₂WO₄) for synthesizing WO₃·H₂O nanoplates. No shape-directing additive was employed. It was observed that the yellow particles were precipitated under aging at 50°C. After aging for 8 h, the precipitated particles were characterized as the WO₃·H₂O nanoplates, and their morphological evolution to square platelet proceeded with an increase of aging time. After aging for 24 h, the WO₃·H₂O square nanoplates were predominantly synthesized. The square nanoplates consisted of a few or several stacked thin layers (thickness, ~10 nm/layer), and provided the well-defined {010} facet for two dominantly exposed surfaces and {101} side facets. Their lateral dimension reached several hundreds of nanometers. It is thus demonstrated that the mild aging (50°C) of the ion-exchanged precursor is a simple and impurity-free synthetic route for WO₃·H₂O square nanoplates. In addition, the monoclinic WO₃ nanoplates were successfully obtained by dehydration-induced topochemical transformation of the WO₃·H₂O nanoplates.