Abstract: This study investigates the effects of laser shock peening (LSP) on the microstructure and mechanical properties of Fe₃₅Co₃₀Ni₃₀Ta₂Mo₂Cu₁ high-entropy alloy (HEA). Compared with the as-cast condition, LSP treatment increases the yield strength and ultimate tensile strength by approximately 320 and 210 MPa, respectively, while maintaining comparable elongation. Microstructural characterization reveals that LSP induces grain refinement, an increase in dislocation density, and the formation of nanoscale Cu-rich precipitates. These features promote dislocation multiplication and improve slip compatibility, reducing strain localization and stress concentration. Furthermore, during deformation, the development of high-angle boundaries (>15°) due to grain subdivision facilitates stress relaxation and sustains ductility. The results elucidate the role of LSP in tailoring hierarchical microstructures to enhance the strength–ductility synergy of HEAs, providing insights for designing high-performance alloys.