A novel cast stainless steel featuring a multiphase microstructure and a nominal composition of Fe-13.5Cr-2.6Si-6.9Ni-1.1Cu-1.1Mn-1.0Mo-0.35Al-0.025C (wt.%) was investigated. Following solution treatment at 1,050 °C and water quenching, the specimens were subjected to further tempering at 570 °C, 610 °C, and 650 °C to explore the effects of critical tempering on microstructure, mechanical properties, and corrosion resistance. Various characterization techniques were employed to examine the phase distribution within the microstructure, with particular attention given to the content and morphology of reverted austenite. Tensile and corrosion tests were carried out to evaluate the performance of the specimens. The results reveal that critical tempering significantly enhances the mechanical properties, with the specimen tempered at 610 °C achieving the highest product of strength and elongation (PSE=23.6 GPa·%), whereas corrosion resistance deteriorates with increasing tempering temperature. Calculations of the martensite start temperature (M_s) and stacking fault energy (γ_SFE) for the reversed austenite in different specimens indicate that the stability of reversed austenite strongly influences mechanical behavior through the TRIP and TWIP effects. However, tempering-induced Cr segregation at ferrite/martensite interfaces and the formation of Cr-depleted zones become more pronounced at higher tempering temperatures, leading to a degradation in corrosion resistance. Furthermore, multiphase coordinated deformation improves the strength-ductility balance, while corrosion tends to initiate at chemically inhomogeneous phase boundaries.