Water plays an important role in the structure and properties of polyelectrolyte-based materials. In this study, the effect of humidity history on the structure and properties of dried polyelectrolyte complexes (PECs) was explored. PECs were assembled from poly(diallyldimethylammonium chloride) and poly(sodium 4-styrenesulfonate) solutions and then dried under controlled humidity conditions. After exposure to higher humidities (humidity tempering), both room temperature storage modulus and flexural modulus of the resulting PEC increased. Water from the humid air plasticized the PEC, increasing mobility and facilitating chain reorganization during humidity tempering, which resulted in a structure with more intrinsic electrostatic bonds (cross-links) and higher moduli. Humidity tempering can achieve a 35% increase in PEC stiffness during room temperature processing with water as the only solvent. Based on these results, humidity tempering is presented as a novel approach to tailoring the structure and mechanical properties of polyelectrolyte-based materials under mild conditions, which makes this approach very appealing to biomaterials and controlled release.