Creep rupture behaviour of dissimilar weld joint (DWJ) between buttered Grade 91 steel (G91) and Alloy-800 fabricated by Shielded Metal Arc Welding (SMAW) process is investigated at 873 K over a stress range of 90–160 MPa. DWJ exhibited lower creep rupture strength than the G91 steel and the failure location of the DWJ shifted with applied stress. At high stresses (≥140 MPa) fracture occurred in the over-tempered region of base metal, whereas it shifted to HAZ at lower stresses. In contrast to interface failures being considered the life-limiting factor in DWJs, the ruptured specimens in the present study revealed a clear interface (between the buttering layer and G91) free from deleterious coarse carbides and insignificant cavitation even after creep exposure of ≈ 5800 h. It’s important to highlight that the buttered G91 DWJ exhibited higher rupture life and weld strength reduction factor compared to joints fabricated without buttering by the SMAW process.