In this study, the thermal-metallurgical behavior of the automatic TIG welding process and post-weld heat treatment (PWHT) at 600 °C for one hour on API 5L X65 high-strength low-alloy pipeline steel was investigated numerically and experimentally. For this purpose, a three-dimensional finite element model using Goldak's double-ellipsoid volumetric heat source was developed in SYSWELD software, and the temperature-dependent properties of the steel were defined. Parallel to the numerical study, actual samples were produced with identical welding parameters and, after applying PWHT, were subjected to metallography and Vickers microhardness tests according to the ASTM E384 standard. The results showed that the melt pool dimensions predicted in the numerical model (based on the 1500 °C melting boundary) with a depth of 2 mm and a width of 6.5 mm accurately match the actual weld cross-section. Additionally, the width and depth of the HAZ region in the simulation were in complete agreement with experimental samples. Evaluations indicated that the model predicts a peak hardness of 450 Vickers for the fusion zone, showing good agreement with the experimental value (460 Vickers). In the as-welded condition, due to the formation of unstable martensite, the hardness exceeds the critical limit of 400 Vickers (according to NACE MR0175/ISO 15156). After applying PWHT, the peak hardness uniformly decreases by approximately 100 units and reaches the safe range of 350 Vickers, which is caused by the tempering of martensite and redistribution of carbon. Ultimately, the consistency between numerical and experimental results confirms the accuracy of the developed model