In the study, the effects of welding parameters on microstructure and mechanical properties of electrical resistance spot welded TBF/DP600 steel sheets of 1.2 mm thickness were investigated. TBF steel used in the study was welded both with and without electro-galvanized. Microhardness measurements and tensile shear tests were taken as basis in determining the mechanical properties, while optical microscope was used for microstructural characterization. The nugget size of the spot-welded samples was determined by image processing technique, while the indentation depths at the electrode pressure points of the samples were measured by ultrasonic technique. It was observed from the fusion zone microstructure images that the two steels did not mix completely. Increasing the welding current and welding time increased the mixing ratio in fusion zone, expanded the heat affected zone and increased the nugget size, indentation depth and load bearing capacity. TBF steel was more affected by the thermal cycle during welding than DP600 steel. At high heat input, liquid metal embrittlement based microcrack formations initiating from the surface in the heat affected zone of galvanized TBF steels were observed, while corrosion started rapidly in the welding region of ungalvanized TBF steels. The highest hardness values were observed in ITAB on the TBF steel side. However, a significant softening occurred in the transition zone between the ITAB and base metal on the TBF steel side. The nugget size, indentation depth and load bearing capacity were found to be relatively higher in the galvanized TBF steel group. In dissimilar electrical resistance spot welded TBF/DP600 steel sheets having the same thickness, DP600 steel with lower strength has determined the welding strength. In high heat input during welding process, fractures are of the fusion zone button type with higher welding strength, while fractures are of partial fusion zone button type with lower welding strength in low heat input. If TBF steel is galvanized, relatively higher welding strength was obtained in the welding parameters that provide relatively lower heat input.