<p>This investigation aims to study the influence of different external cooling media on the microstructural evolution and mechanical performance of AZ31B magnesium alloy during friction stir processing (FSP). The process was carried out under four cooling conditions—normal air, compressed air, CO₂, and water—to tailor the thermal cycle and promote grain refinement. The FSP parameters were fixed at 1500 rpm, 78 mm/min, and 2° tool tilt. A significant amount of grain refinement was observed in the microstructural studies of all cooling assisted FSP samples compared to the base metal. Among all, CO₂ cooling exhibited the finest grain size of 7.01 ± 0.56 µm, with 54.5 % reduction relative to other cooling conditions. Due to the controlled cooling rate, post-dynamic recrystallization grain growth was effectively suppressed. Mechanical testing showed consistent improvement across the cooled samples, and depth-wise microhardness profiles indicated uniform stirring and defect-free consolidation. CO₂ cooling gave the best balance of microstructural integrity and mechanical performance across all the cooling settings, highlighting the importance of controlled cooling in enhancing the properties of FSP-processed magnesium alloys.</p>