Subduction-zone fluids play a critical role in mass transfer and crust-mantle evolution at convergent plate margins, yet their exact sources and fluid-rock interaction processes remain debated. Jadeitites in forearc serpentinite mélanges serve as unique archives for these fluids. Here, we present high-precision Mg–Fe isotopic data for jadeitites and jadeite-rich rocks from the Rio San Juan Complex (RSJC), Dominican Republic and integrate data from worldwide jadeitites to constrain fluid sources and geochemical evolution in the forearc mantle wedge. RSJC jadeitites exhibit low δ26Mg values (from −0.92‰ to −0.16‰, average = −0.48 ± 0.32‰, 2SD, n = 21) that lack correlations with carbonate indicators, precluding a significant carbonate contribution. Instead, the coupling of light Mg isotopes with enriched Ni and Cr contents, together with systematic negative correlations among δ26Mg, Na2O/TiO2, MgO and Ni, fingerprints a substantial contribution from serpentinizing fluids at forearc depths. In contrast to the light Fe isotope signatures of Myanmar jadeitites, RSJC jadeitites display relatively high δ56Fe values (−0.08‰ to 0.29‰, average = 0.15 ± 0.16‰, 2SD, n = 20). Systematic covariations between δ56Fe values, TFe2O3 contents and redox-sensitive proxies (V/Sc, U/Th, Ce anomalies and Sb/As) in jadeitites from the RSJC and Myanmar indicate that Fe isotope heterogeneity is primarily controlled by the redox state of the fluids. By integrating petrological and geochemical constraints, we propose that forearc serpentinization acts as a critical redox filter that governs the coupled Mg–Fe isotope heterogeneity of jadeitites. Olivine-dominated serpentinization generates reducing conditions and promotes Fe isotope fractionation, producing light Fe isotopic signatures as recorded by Myanmar jadeitites. In contrast, serpentinization involving orthopyroxene buffers the system under relatively oxidizing conditions, preserving high δ56Fe values in RSJC jadeitites. Jadeitite-forming fluids are best explained as mixtures of AOC-derived and serpentinizing fluid components, which either directly precipitate P-type jadeitites or metasomatize igneous protoliths to form R-type jadeitites. The subduction and melting of these rocks may transfer light Mg and heterogeneous Fe isotopic signatures to the mantle wedge and related arc magmas. Forearc serpentinization thus exerts critical control on the redox state and chemical evolution of subduction-zone fluids, with important implications for the chemical heterogeneity of the mantle wedge.