Reinforced concrete (RC) beams are fundamental structural components that leverage the compressive strength of concrete and the tensile strength of steel. However, reinforcement corrosion, exacerbated by environmental factors such as moisture infiltration, seawater exposure, and insufficient protective cover, severely compromises their structural integrity. While previous research has predominantly relied on experimental techniques, this study employs Finite Element Method (FEM) simulations using ANSYS software to assess the impact of varying corrosion levels on RC beam performance. Four beam models (1600 mm × 230 mm × 140 mm) were analysed at corrosion rates of 0%, 10%, 20%, and 30%, with results validated against experimental data. The simulation revealed maximum deviations in load–deflection values of 17.08%, 5.66%, 6.57%, and 6.69%, respectively, with moment–rotation curves demonstrating strong alignment with experimental findings. A subsequent parametric study at a 50% corrosion level predicted a load capacity of 74.7 kN at 15 mm deflection, reinforcing the trend of progressive load capacity reduction with increasing corrosion. The study highlights the significant deterioration in structural performance due to corrosion-induced material loss, particularly at higher corrosion levels. These findings confirm the effectiveness of FEM simulations as a reliable and cost-efficient alternative to traditional experimental approaches. The research underscores the critical importance of addressing corrosion in ageing infrastructure, providing crucial insights into the long-term performance and safety of RC structures exposed to severe deterioration.