In the present investigation, a detailed microstructure and mechanical behaviour study for P91 welded joint were investigated. The welded joint was produced using the Gas Tungsten Arc welding (GTAW) process with dissimilar Ni-based filler IN625 (ERNiCrMo-3). For microstructure characterization of the weld joint, optical microscopy and scanning electron microscopy were utilized. The effort was made to get a relationship between microstructure and mechanical properties variation. The residual stress measurement was also carried out for both the condition by using the hole-drilling method. To evaluate the mechanical properties, tensile testing and Charpy testing was done at room temperature. The weld joint developed with dissimilar Ni-based filler processes the austenitic microstructure having equiaxed dendrites in bulk weld metal and columnar and cellular dendritic structure near the interface. The segregation of the alloying elements like Mo and Nb was also confirmed along the inter-dendritic areas. The welded joint qualified the tensile test and Charpy test with average value of 580 ± 12 MPa and 65 ± 5 J, respectively in as-welded. However, a reduction was observed in tensile strength and Charpy toughness after the PWHT. A significant variation in microhardness was measured across the welds joint, and peak hardness was measured about 344 HV in the coarse-grained heat affected region while in weld metal it was 263 HV. The peak hardness value in HAZ and variation in hardness also get reduced after the PWHT. In the weld metal, a significant variation in the residual stresses was measured along the thickness with peak magnitude 250 MPa and 172 MPa for longitudinal and transverse stress, respectively in the as-welded condition. A significant reduction in peak residual stresses (longitudinal stress and transverse stress) was obtained after the PWHT. © 2021 Elsevier Ltd