The aim of the present study is to reduce the width of the heat affected zone of the 2.25Cr-1Mo steel weldment by introducing the friction stir welding process. In FSW process, the tool rpm is the deciding factor for the evolution of peak temperature and cooling rate, which decides the HAZ width. Consequently, the FSW process was carried out with two different tool rpm via 200 and 500 rpm. Microstructural evolution and mechanical properties of the FSW weldment are studied in detail and compared to that of the A-TIG welded joint. The FSW weld metal exhibited higher hardness (405 ± 3HV0.2 for 500 rpm and 370 ± 3HV0.2 for 200 rpm) compared to that of the A-TIG weld (325 ± 4HV0.2) due to the substantial grain refinement and martensitic phase transformation. After post-weld heat treatment at 1003 K for an hour, a gradual reduction in the hardness in the weld was observed. The tensile strength of the FSW joint made with the rotational speed of 500 rpm (554 ± 8 MPa) and 200 rpm (545 ± 5 MPa) was more or less equal to that of the A-TIG (552 ± 7 MPa) weldment. Moreover, FSW weldment exhibits lower HAZ width compared to A-TIG weld. Based on this study, it is recommended that the FSW process employing W-La2O3 tool can be successfully employed as an alternative technique for fusion welding of 2.25Cr-1Mo steel.