Abstract: The mechanism of the effect of the heat input in TIG welding on the microstructure and hardness of 1.2 mm-thick 2507 super duplex stainless steel (DSS) welded joints was studied with micro-examination, chemical composition analysis, hardness measurement, and electron backscatter diffraction (EBSD) analysis. According to the results, in the weld seam, as the heat input increased from 110 J/mm to 156 J/mm, the ferrite grain size increased from 90 μm to 200 μm. Grain coarsening reduced the nucleation sites of austenite. In the meantime, increased heat input reduced the N content in the weld seam from 0.25% to 0.21%, eventually resulting in a decrease of the austenite content from 28.9% to 24.3%. In the high temperature heat-affected zone (HTHAZ), the austenite content peaked at 36.4% when the heat input was 132 J/mm, at which point the Cr₂N precipitation amount at the center of ferrite grains bottomed. When the welding heat input increased from 110 J/mm to 156 J/mm, the Vickers hardness of the weld seam remained at 320 HV. The reason behind is that both the geometrically necessary dislocation density and phase boundary density of the weld seam remain unchanged with the increase of heat input. According to the metal reinforcement theory, weld hardness won’t change as heat input increases.