Abstract: The automotive industry’s trend towards lightweighting has led to a widespread usage of high-strength steels (HSS), which poses challenges for resistance spot welding (RSW) process in auto body manufacturing. One such challenge is the frequent occurrence of the initial gap (IG), which can negatively impact the consistency of the RSW process for HSS. This study aims to reveal this impact by comparing multi-sensor process signals, weld surface morphology, nugget size, and its growth process under standard and two-sided IG conditions. A comprehensive analysis of energy input and contact status is performed to investigate the influence mechanism of IG condition on nugget growth and process signal evolution. The study found that the IG disturbance reduces the initial contact area of the sheet-to-sheet interface in comparison to the standard condition. This results in a faster rise in the sheet temperature, an earlier peak in the resistance signal, and a greater susceptibility to expulsion at the early stage of the welding process. During the subsequent process, there is a significant increase in the contact area of both sheet-to-sheet and electrode-to-sheet interfaces, leading to a decrease in dynamic resistance signal and heat generation. Consequently, the nugget size and electrode displacement signal are smaller than the standard ones. Furthermore, the larger contact area along the gap constraint direction causes more heat generation, ultimately resulting in a larger nugget dimension and indentation size in this particular direction. This research can provide guidance for online monitoring and control for the RSW process of HSS.