<p>This study provides a numerical assessment of a key parameter governing the seismic performance of steel beam-to-circular column connections that utilize a steel box and stiffeners. As a first step, the numerical model was validated against the experimental specimen, and the influence of the stiffener length on the steel box-to-column joint was investigated. The cyclic analyses were conducted to evaluate the influence of the stiffener length-to-column diameter ratio on the connection performance. Subsequently, the effect of this parameter on the ultimate moment and elastic stiffness, was assessed. Based on curve fitting, approximate equations were also introduced to calculate the ultimate moment and elastic stiffness. Finally, the connection rigidity was evaluated according to the AISC code. The results show that increasing the stiffener length leads to a significant reduction in connection stiffness and ultimate moment. This stiffness reduction reached 51% in models with larger sections. Furthermore, in models with larger beam sections, the impact of stiffener length was more pronounced compared to those with smaller sections. The developed analytical expressions for assessing the ultimate moment and elastic stiffness accurately forecast the results for these connections.</p>