Serpentinite-derived fluids are pivotal for fluid mobility and mass transfer in subduction zones. However, the redox state of these fluids and their effects on the composition of continental crust remain unclear. Here, we investigated the Fe3+/∑Fe ratios and Fe isotope compositions of high-pressure fluid-metasomatized schists and blackwall zones surrounding serpentinites from the Tauern Window, Eastern Alps. Relative to the surrounding granodiorite protolith (δ56Fe: −0.14 to 0.66‰), schists in the Stillup Tal shear zone exhibit significantly lower δ56Fe values (−0.25 to −0.11‰), higher Fe−Mg contents and lower concentrations of large ion lithophile elements. These observations are consistent with the chlorite blackwall rimming a nearby serpentinite body in the Pfitsch area, both of which can be ascribed to the metasomatism by serpentinite-derived fluids. Furthermore, the schists display substantially lower Fe3+/∑Fe ratios than granitoid protoliths. A strong positive correlation between Fe3+/∑Fe and V/Sc ratios of the schists suggests that serpentinite dehydration released reduced fluids. Considering the mineralogical and geochemical differences between the central chlorite-rich schist and other schists in the Stillup Tal shear zone, we propose a fluid metasomatism model for the formation of metasomatic rocks. Initially, serpentinite-derived fluids migrated along a major shear zone, metasomatizing granitoids to form central chlorite-rich schists with extremely high Fe contents and low δ56Fe values (−0.25‰) with high fluid/rock ratios. Subsequently, the evolved serpentinite-derived fluids continued their migration along secondary fractures, generating schists with moderately low δ56Fe values (−0.24 to −0.11‰) under reduced fluid/rock ratios. Our results provide geochemical evidence for the reduced property of serpentinite-derived fluids and their impact on the redox state and Fe isotope composition of rocks with which they interact.