Serpentinite-derived fluids have been invoked to explain the isotopically light Fe signatures and elevated oxygen fugacity of arc magmas relative to MORB. However, direct natural constraints on the Fe isotope signature of slab-derived fluids remain elusive, because dehydration of serpentinite typically occurs under low effective fluid/rock ratios and the residual rocks largely buffer bulk isotopic signals. Here we investigate metasomatized dikes enclosed within subducted serpentinite bodies that can serve as sensitive recorders of fluid compositions under high-pressure conditions. We present a comparative Fe isotope study of eclogitic-facies metarodingites and associated metagabbro blocks from the Voltri Massif in the Ligurian Alps, together with low-grade rodingites from the Northern Apennines that escaped Alpine subduction. All samples are derived from a common Fe–Ti gabbroic protoliths, allowing isolation of subduction-related effects. Low-grade rodingites preserve magmatic δ56Fe signatures, demonstrating that seafloor rodingitization does not produce resolvable Fe isotope fractionation. The metagabbros show a slight shift toward lighter composition, whereas the metarodingites display a systematic and more pronounced decrease in δ56Fe. Petrological evidence for syn-metamorphic veining supports fluid infiltration. The retention of FeO/TiO2 ratios excludes obvious Fe gain or loss, and the depletion of fluid-mobile elements rules out interaction with crustal fluids. These observations are best explained by isotopic exchange with externally derived serpentinite fluids during subduction. Open-system fluid−rock modelling demonstrates that the observed isotopic shift requires high integrated fluid/rock ratios, which is achievable in thin dikes surrounded by voluminous serpentinites. Integrating field constraints and internally consistent fractionation factors, we estimate that serpentinite-derived fluids at 500 ℃ possess δ56Fe values of −0.17 to −0.04‰ when Fe is transported by Cl−- and SO42−- bearing complexes. These findings provide direct natural evidence that serpentinite dehydration releases isotopically light Fe-bearing fluids into subduction zones, offering support for the model linking slab-derived fluids to the Fe isotope budget of the mantle wedge.