In situ post weld heat treatment (PWHT) by applying a second pulse current during resistance spot welding (RSW) provides a new pathway to alter the microstructure of the fusion zone (FZ) and improves the mechanical performance of the RSW joint. In the present study, effect of the second pulse current on microstructural characteristics and mechanical performance of resistance spot weld joint of HSLA350 steel under were investigated. It was observed that after applying the second pulse current during welding process, it subdivides the initial solidified fusion zone into two zones, namely equiaxed grain zone (EGZ) and columnar grain zone (CGZ). The outer layer becomes EGZ consisting of quasi-equiaxed grains of ferrite and martensite whereas the inner core is CGZ solidified with a columnar grain structure consisting of martensite and some bainite. The refinement of microstructure in the case of double pulse weld resulted in enhanced tensile shear strength and failure energy absorption capacity with ductile pullout failure mode. © 2019, Springer-Verlag London Ltd., part of Springer Nature.