This study employed the friction stir welding technique to find the optimal tool traverse speed for successfully joining the 304 stainless steel with 6061-T6 aluminum alloy. For this purpose, five different traverse speeds at a constant rotational speed of 875 rpm were considered. The welded joints were characterized based on the mechanical properties, micro/macrographs, and quantification of steel fragments and intermetallic compounds (IMCs) inside the stir zone (SZ). The composition and phase of the IMCs at the interface, SZ, and intercalated structure were confirmed through X-ray diffraction analysis, energy-dispersive X-ray spectroscopy, field emission scanning electron microscope, and atomic force microscope. It is observed that the traverse speed of 90 mm/min produced the highest tensile strength with an IMC layer thickness of 1.47 ± 0.23 μm. At this traverse speed, the area fraction of IMCs and steel fragments is around 0.023 % and 6.05 % of the SZ area, respectively. Furthermore, the defects increased by reducing the traverse speed from 90 to 30 mm/min because of a higher fraction of IMCs (i.e., Al3Fe) and steel fragments inside the SZ. The traverse speed higher than 90 mm/min generated the cracks at the interface because of insufficient stirring at low heat input. The hardness improved by decreasing the traverse speed because of grain refinement in SZ and thermomechanically affected zone. The intercalated structure acted as the hardest zone because of the formation of IMCs, namely Al3Fe + AlFe at 30–70 mm/min, and Al3Fe + AlFe + AlFe3 at 90 and 120 mm/min.