This study presents a novel strengthening approach in a ferrous medium-entropy alloy (Fe-MEA) through periodic spinodal decomposition. By introducing Cu and Al into the alloy, the researchers achieved a significant enhancement in mechanical strength without compromising ductility. The matrix underwent nanoscale periodic spinodal decomposition via a simple one-step aging process, leading to a doubling of strength while preserving elongation. The periodic spinodal decomposed structures effectively mitigated strain localization, enhancing both strength and ductility. Spinodal decomposition, which creates finely distributed coherent structures, offers high versatility due to minimal elemental addition, making it a promising method for improving mechanical properties of various alloys. The Fe-MEA was designed with a complex composition to induce spinodal decomposition, increasing entropy and enthalpy to enhance the probability of decomposition. The alloy contained a miscibility gap, enabling spinodal decomposition. The study revealed the formation of nanoprecipitates such as Fe₂SiTi, Ni₃Ti, and B2, along with Cu-rich FCC phases. The periodic spinodal decomposition induced a coherency strain, generating internal stress that inhibited dislocation motion, thereby enhancing strength without reducing ductility. The mechanical properties of the aged sample showed a yield strength of 1090 MPa, ultimate tensile strength of 1310 MPa, and elongation of 28.5%, demonstrating a significant improvement in strength-ductility balance. The strengthening mechanism involved spinodal hardening, which contributed more significantly than precipitation hardening. The periodic spinodal decomposition structure hindered dislocation motion, leading to wavy and serrated dislocations, and enhanced work hardening. The study highlights the potential of spinodal decomposition as an effective strategy for improving the mechanical properties of advanced alloys, offering a new approach to balance strength and ductility in high-entropy alloys. The results demonstrate that spinodal decomposition can be a promising method for developing next-generation metallic materials with enhanced performance.This study presents a novel strengthening approach in a ferrous medium-entropy alloy (Fe-MEA) through periodic spinodal decomposition. By introducing Cu and Al into the alloy, the researchers achieved a significant enhancement in mechanical strength without compromising ductility. The matrix underwent nanoscale periodic spinodal decomposition via a simple one-step aging process, leading to a doubling of strength while preserving elongation. The periodic spinodal decomposed structures effectively mitigated strain localization, enhancing both strength and ductility. Spinodal decomposition, which creates finely distributed coherent structures, offers high versatility due to minimal elemental addition, making it a promising method for improving mechanical properties of various alloys. The Fe-MEA was designed with a complex composition to induce spinodal decomposition, increasing entropy and enthalpy to enhance the probability of decomposition. The alloy contained a miscibility gap, enabling spinodal decomposition. The study revealed the formation of nanoprecipitates such as Fe₂SiTi, Ni₃Ti, and B2, along with Cu-rich FCC phases. The periodic spinodal decomposition induced a coherency strain, generating internal stress that inhibited dislocation motion, thereby enhancing strength without reducing ductility. The mechanical properties of the aged sample showed a yield strength of 1090 MPa, ultimate tensile strength of 1310 MPa, and elongation of 28.5%, demonstrating a significant improvement in strength-ductility balance. The strengthening mechanism involved spinodal hardening, which contributed more significantly than precipitation hardening. The periodic spinodal decomposition structure hindered dislocation motion, leading to wavy and serrated dislocations, and enhanced work hardening. The study highlights the potential of spinodal decomposition as an effective strategy for improving the mechanical properties of advanced alloys, offering a new approach to balance strength and ductility in high-entropy alloys. The results demonstrate that spinodal decomposition can be a promising method for developing next-generation metallic materials with enhanced performance.