Abstract This paper aims at investigating metallurgical and mechanical characterization of dissimilar laser spot welds between low carbon and austenitic stainless steel sheets. Microstructural examination, microhardness test and quasi-static tensile–shear test were performed. Mechanical properties of the welds were described in terms of peak load. The effects of laser mean power on the performance of dissimilar laser spot welds have been studied. It was found that increasing laser mean power leads to the transition of laser welding mode from conduction to keyhole. This transition causes a significant growth of the fusion zone size in the lower sheet, i.e. the low carbon steel sheet; since, the keyhole acts as an effective trap for the laser beam and will greatly increase the energy absorption from the incident laser beam. It is also shown that the fusion zone size in the weaker sheet, i.e. the low carbon steel sheet is the controlling factors in determination of the mechanical strength of dissimilar austenitic/ferritic laser spot welds.