Abstract: Objective The aim is to study pitting corrosion resistance and mechanism of welded joints of 254SMO super austenitic stainless steel. Methods ERNiCrMo-3 solid welding wire with a diameter of ϕ1.2 mm was used for MIG welding of 254SMO super austenitic stainless steel plates. Pitting corrosion resistance of welded joints was evaluated, and optical microscopy, scanning electron microscope (SEM), energy dispersive spectrometer (EDS), electron backscattered diffraction (EBSD), electron probe microanalysis (EPMA) and other methods were employed to analyze microstructure and corrosion mechanism of welded joints. Results The results show that pitting corrosion resistance of 254SMO welded joints is significantly inferior to that of base metal. An unmixed zone (UMZ) with a thickness of approximately 50 μm exists in MIG weld of 254SMO super austenitic stainless steel. Although chemical composition of UMZ is identical to that of base metal, it undergoes melting and resolidification, leading to severe segregation of Mo element. The minimum Mo content in UMZ is 4%, far lower than 6% in base metal, which significantly degrades pitting corrosion resistance of UMZ. Conclusion UMZ of 254SMO welded joints undergoes pitting corrosion prior to base metal and weld metal, forming corrosion grooves.