This study proposes a method to improve the mechanical strength of martensitic welded joints by minimizing the effect ofmartensitic tempering. The high-strength martensitic steel plate exhibiting a tensile strength of 1500 MPa was successfullyjoined in this study utilizing additional water-cooling by friction stir welding (FSW) technique. Concurrently, conventionalair-cooled welded joints were produced utilizing identical welding parameters for comparison. Notwithstanding the macrostructureof both FSW joints was similar, the air-cooled joint contained a larger stir zone (SZ) and wider heat-affectedzone (HAZ). The air-cooled joint SZ exhibited a microstructure featuring coarse martensite, while the water-cooled jointSZ displayed a microstructure consisting of fine martensite. The degree of self-tempering of the martensite in the HAZ wassignificantly reduced, together with fewer and finer carbide particles than in the HAZ of air-cooled joints, by applying watercooling. The tensile test results indicated that the water-cooled FSW joint showed a significant improvement (23.6%) inultimate tensile strength compared to its air-cooled counterparts.