The widespread use of stainless steels in various industries due to their appropriate strength and corrosion resistance necessitates the investigation of dissimilar joints between them. However, resistance spot welding (RSW) of AISI304 austenitic stainless steel (ASS304) to AISI420 martensitic stainless steel (MSS420), particularly in case of employing interlayer, has not been examined so far. Due to appreciable difference in their physical properties and metallurgical consistency as main concerns, the effect of FeCoNiCuAl0.2 high-entropy alloy (HEA) interlayer on the microstructural and mechanical behavior of the joints was investigated. It was found that copper exhibited a segregation in the HEA cast ingot because of its positive mixing enthalpy with other elements. This issue was responsible for the formation of hot cracking in the weld nugget, resulting in a decrease in mechanical strength of the joints. The effect of welding current on the joint strength was investigated as well. The relatively suitable current intensity in this joint was found to be 7 kA, and the maximum failure force was achieved in this case at about 6000 N. At lower current intensity (6 kA) due to insufficient heat input, proper fusion between the base metals and the interlayer was not formed and the maximum fracture force was about 4000 N. At 8 kA current intensity, due to higher heat input and increased weld nugget size, a higher fracture force was obtained than at 6 kA current intensity, but the displacement was reduced. The decrease in fracture toughness at 8 kA current intensity was attributed to spattering and increased copper segregation due to higher temperature (more diffusion of copper atoms).