Dissimilar welds between austenitic stainless steel Super304H and ferritic-martensitic steel T92 were produced using hot-wire gas tungsten arc welding employing ErNiCrFe-7A fillers. The welds were post-weld heat-treated (PWHT) at 760 °C for 2 h and subjected to creep testing at 650 °C and 120 MPa. The weld microstructures were characterized in detail in the as-welded, PWHTed, and crept conditions. The creep failures were observed to take place in the fine-grained heat-affected zone (FGHAZ) of alloy T92. Microstructural studies clearly revealed accelerated degeneration of martensitic lath structure, coarsening of chromium-rich carbides, and formation of detrimental Laves phase in the FGHAZ of alloy T92. The reasons for the accelerated microstructural degradation in the FGHAZ of alloy T92 are discussed in detail.