Buckling-restrained braces (BRBs) are widely recognized as one of the most effective seismic resisting systems due to their high energy dissipation capacity and stable hysteretic behavior. However, the use of concrete or mortar filler materials in conventional BRBs increases structural weight, fabrication complexity, and construction costs. Therefore, the development of all-steel BRBs without filler material offers a promising alternative for simplifying fabrication and improving practical applicability. In this study, the seismic performance of all-steel buckling-restrained braces without filler material was investigated through finite element modeling using ABAQUS. The numerical model was first validated against available experimental results and subsequently employed in a comprehensive parametric study. The effects of key geometric parameters, including core cross-sectional dimensions, core width-to-thickness ratio, and the clearance between the core and the restraining member, were evaluated under cyclic loading conditions. The results demonstrated that all-steel BRBs without filler material are capable of providing stable cyclic behavior, adequate ductility, and considerable energy dissipation capacity. Furthermore, increasing the core thickness enhanced the load-carrying capacity and improved the overall cyclic performance of the brace. The findings also indicated that reducing the core width-to-thickness ratio from 8 to approximately 5 significantly improved seismic performance, with the optimum ratio occurring near 5. In addition, excessive clearance between the core and the restraining member led to reductions in both strength and energy dissipation capacity, whereas clearances smaller than 3 mm resulted in more favorable behavior. The main contribution of this study lies in the comprehensive evaluation of the combined effects of critical geometric parameters on the seismic response of filler-free all-steel BRBs and the proposal of practical design ranges for improving their structural performance.