This study presents a comprehensive numerical investigation aimed at evaluating the effectiveness of High Performance Fiber Reinforced Cementitious Composite (HPFRCC) sheets in strengthening beam–column joints of reinforced concrete (RC) frames. The HPFRCC used in this research was first produced through a controlled laboratory fabrication process, and its mechanical properties were characterized via tensile, compressive, and flexural tests to ensure high fidelity material data for numerical modeling. A three dimensional frame finite element (FE) model incorporating nonlinear constitutive laws for concrete, reinforcing bars, and HPFRCC was then developed and validated against reliable experimental results. Subsequently, the influence of HPFRCC sheet length in three strengthening configurations—side, bottom, and combined—was examined under monotonic loading. The findings indicate that increasing the sheet length leads to notable improvements in peak load resistance and initial effective stiffness. The strengthened joints exhibit more favorable stress redistribution within the core region, reduced tensile stress concentrations, and significantly lower cumulative concrete damage. Among the evaluated configurations, the combined strengthening scheme provides the highest enhancement, resulting in a 33–59% increase in load bearing capacity and up to a 72% increase in energy absorption capacity compared to the unretrofitted reference joint. Overall, the results demonstrate that strategically applied HPFRCC sheets constitute an efficient and reliable method for enhancing the performance of RC beam–column joints.