The objective of the current investigation to carry out a detailed research study on the relationship of microstructure and mechanical properties of the dissimilar Gas Tungsten Arc Weld (GTAW) joint of heat-resistant ferritic/martensitic P91 and austenitic SS304H steel, prepared using the Ni-grade ERNiCrMo-3 filler. The metallographic characterization of the weldments (P91 HAZ, weld metal, SS304H HAZ) was performed by availing the optical microscope and scanning electron microscope equipped with energy dispersive spectroscopy for precipitate size, their distribution in matrix, morphology and chemical composition. Mechanical properties of the weldments were also evaluated and for that, tensile testing, microhardness testing and Charpy testing were conducted. The metallographic and mechanical characterization of the weldments were also performed after the post-weld heat treatment (PWHT) at 775 °C for 60 min and compared with the as-welded condition. The results indicated that a complete heterogeneous microstructure formation across the weldments which led to variation in the mechanical behaviour of the weldments. The tensile strength and Charpy impact toughness (CIT) of the weld metal were measured 631 MPa and 79 J, respectively, in the as-welded condition. The coarse-grained HAZ region of P91 side showed the peak hardness of 412 HV (as-welded), which was higher than the hardness of the weld metal (303 HV). After the PWHT, an improvement in tensile properties was observed and tensile strength and % elongation was measured 666 MPa and 16.14 %, respectively. After the detailed study, it was concluded that the GTAW process with ERNiCrMo-3 filler offered a better combination of the microstructure and mechanical properties in as-welded condition for the dissimilar welded joint of P91 and SS304H. © 2021 The Society of Manufacturing Engineers