Abstract: To investigate the effect of welding methods on the lap joint performance of 304/201/304 stainless steel composite plates, three welding methods, namely resistance welding, laser welding, and metal inert gas (MIG) welding, were applied to lap-weld the rolled composite plates. The macroscopic weld morphology, microstructure, elemental distribution, grain size, and grain boundary characteristics were analyzed by means of optical microscopy, scanning electron microscopy, and electron backscatter diffraction, and the mechanical properties were evaluated by microhardness testing, nanoindentation, tensile fractography, and 180° bending tests. The results show that all three welding methods produce joints without macroscopic defects, but the weld morphology, joint microstructure, and properties differ. Resistance welding and laser welding have low heat input and fast cooling, resulting in fine equiaxed dendrites in the weld center with average grain sizes of 10.6 μm and 10.1 μm, respectively, and fine columnar grains at the weld edge. MIG welding has high heat input and slow cooling, resulting in coarse columnar dendrites in the weld center with an average grain size of 27.6 μm. In terms of elemental distribution, resistance welding exhibits the highest Mn content due to self-melting of the base metal, laser welding shows the lowest Mn content due to the addition of a low-Mn filler wire, and MIG falls between the two. Regarding mechanical properties, the resistance welded joint has the highest hardness, with an average microhardness of 258.8 HV in the weld zone and a nanohardness of 5.15 GPa at the weld center, which is mainly attributed to fine-grain strengthening, high dislocation density, and Mn solid-solution strengthening, but its plasticity is the poorest. The laser welded joint ranks second, with average values of 255.7 HV and 5.11 GPa, achieving the best balance between strength and plasticity. The MIG joint has the lowest hardness (244.1 HV and 5.01 GPa) but exhibits the best plasticity. Bending tests show that all three joints can withstand 180° bending and have good bending resistance. This study provides a theoretical basis and technical support for selecting welding methods for stainless steel composite plates used in automotive fuel systems.