Abstract: P91 steel, as a typical new martensitic heat-resistant steel, has been widely used in nuclear power plants for main steam and main feedwater circuits due to its low coefficient of thermal expansion and excellent high-temperature creep resistance, demonstrating significant advantages over traditional low-alloy high-strength steels. This study focuses on P91 steel pipes with dimensions of Φ323.8 mm × 25.4 mm. The root welding was performed using the GTAW (Gas Tungsten Arc Welding) process, while the filler welding was carried out using three different processes: SMAW (Shielded Metal Arc Welding), SAW (Submerged Arc Welding), and automated GTAW. Post-weld heat treatments were applied to the welded joints at two different peak temperature ranges—300°C–350°C and 740°C–760°C—to relieve residual stresses. The hole-drilling method was employed to measure the residual stresses at different locations of the welded joints under heat-treated conditions. A systematic comparison was conducted to analyze the influence of different welding processes and peak temperature heat treatments on the residual stresses in P91 steel welded joints. The results indicated that the GTAW+SMAW process combination resulted in the highest residual stresses, followed by GTAW+SAW, while GTAW+automated GTAW produced the lowest residual stresses. Additionally, the high-temperature heat treatment was significantly less effective in reducing residual stresses compared to the low-temperature heat treatment. The findings provide a theoretical basis for reducing internal welding residual stresses in components and enhancing their service life.