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The effect of heat input on the microstructure and mechanical properties of dissimilar welds of direct-quenched ultrahigh strength steel (Optim 960 QC) and duplex stainless steel (UNS S32205) was studied. The effect of heat input on grain coarsening and the proportion of bainite-martensite on the ferritic side and ferrite-austenite on the duplex side was clearly evident. The hardness profile showed a trend of increasing hardness with lower heat input. Enhancement in tensile strength corresponded to lower heat input and increase in hardness. The elongation values and bend behavior were roughly the same for all the specimens; there was no clear link to the heat input. Moderate heat input gave optimum impact toughness in the weld and the ferritic HAZ. The fatigue performance of the specimens demonstrated the profound influence of geometrical effects. However, the effect of microstructural characteristics on fracture location was also observed.