The water-to-cement ratio and temperature are key factors governing ionic transport in cement-based materials. However, quantitative descriptions of their coupled effects on sulfate ion transport–reaction processes remain limited. In this study, a coupled transport–reaction model is developed to simulate sulfate ingress in cementitious materials subjected to external sulfate attack. The predictive capability of the model is validated through a series of well-designed experiments, and the distributions of sulfate ion concentration within the materials under different water-to-cement ratios and temperature conditions are systematically analyzed. The results demonstrate good agreement between model predictions and experimental measurements, indicating that increases in both water-to-cement ratio and temperature significantly accelerate sulfate diffusion and intensify chemical reactions between sulfate ions and cement hydration products, thereby promoting material degradation. These findings highlight that the coupled effects of water-to-cement ratio and temperature should be explicitly considered in durability assessment and numerical modeling of external sulfate attack in cement-based materials to improve predictive accuracy.