Laser welding of thick-section 316L stainless steel is a promising manufacturing process for fusion power generation cooling components, where weld integrity is essential for reliable operation under extreme service conditions. This study investigates the influence of different shielding gases on key weld quality parameters for welding 5 and 16 mm thick 316L. Three shielding gases, argon, nitrogen and helium, were evaluated to determine their effects on laser energy absorption, spatter behaviour, porosity formation and surface oxidation. For 5 mm welds, helium produced the greatest weld profiles, attributed to its high ionisation potential and thermal conductivity promoting improved laser energy coupling through plume suppression. However, no significant differences in laser energy absorption were found at 16 mm due to other overpowering welding phenomena. High-speed imaging combined with image processing demonstrated that spatter increased with laser power and plate thickness, with helium consistently producing the lowest spatter levels. At 9 kW and 16 mm, helium reduced spatter by 54% compared with nitrogen. Nitrogen shielding resulted in the lowest porosity across both plate thicknesses owing to its high solubility and reactivity within the molten weld pool, which facilitated gaseous escape. Surface oxide analysis showed that argon produced the thinnest Cr₂O₃ layer (1.3 µm), followed by nitrogen (2.45 µm) and helium (2.6 µm), reflecting argon's higher density for surface protection. These findings demonstrate shielding gas selection strongly influences defects and weld quality in thick-section laser welding, highlighting the need for a tailored shielding strategy depending on the targeted weld performance requirements in fusion applications.