This study presents a comprehensive numerical investigation of a novel structural energy dissipation device, called S-shaped steel damper (SSSD). SSSD is innovatively designed by bolting together two conventional U-shaped steel dampers, triggering -when subjected to shear- a composite mechanism that enhances energy dissipation while maintaining low effective horizontal stiffness. To evaluate its structural performance, a prototype of SSSD was subjected to in-plane cyclic loading tests, demonstrating stable hysteretic behavior with substantial energy dissipation capacity and minimal strength degradation. Finite element (FE) analyses were performed using ABAQUS to further elucidate the influence of key design parameters on the damper mechanical response. While the in-plane behavior exhibited excellent energy dissipation capacity, the out-of-plane performance was significantly enhanced by in-creasing the damper width, leading to improved overall damping capacity. Furthermore, the effects of geometric modifications, including variations in damper thickness, width, and bolt spacing, were systematically analyzed under both in-plane and out-of-plane loading conditions. The results revealed distinct differences in stiffness and energy dissipation characteristics between the two loading directions, with the in-plane configuration demonstrating superior stability and efficiency. In addition, this study identifies critical design optimizations to enhance the out-of-plane stiffness and energy dissipation capacity of the SSSD. The findings provide valuable insights for engineers and researchers, serving as a foundation for the refined design and broader application of SSSDs in seismic retrofitting and structural vibration control.