Nickel-based superalloy Inconel 718 and austenitic stainless steel AISI 304 are joined by rotary friction welding. The changes in microstructure and mechanical properties of the dissimilar joint for solutionising, solutionising + aging, and post-weld aging treatment, with a prior solution heat treatment, are studied. Two distinct microstructural zones, the FDZ (fully deformed zone) and the TMAZ (thermo-mechanically affected zone) are observed in all weldments. The post-weld aged joint sample shows the highest tensile strength and hardness due to the precipitation of γ′ (Ni3(Ti, Al)) and γ″ (Ni3Nb) strengthening precipitates. The solution-treated sample shows the highest yield strength, while the solutionising + aging treatment is found to deteriorate the mechanical properties of the dissimilar joint. The observed changes in the mechanical properties are investigated based on microstructure and electron dispersive spectroscopy.