Biphasic High‐Entropy Heterojunctions Enabled by Perovskite Transformation

High‐entropy materials (HEMs) provide an ideal platform for tailoring functional properties. However, the controlled synthesis of biphasic high‐entropy heterostructures with synergistic multifunctionality remains a significant challenge. We propose an in‐situ chemical reduction strategy driven by dynamic transformations, enabling the conversion of perovskite precursors into a series of anchored high‐entropy heterojunctions. Under reducing conditions, the entropy‐stabilized perovskite lattice acts simultaneously as both a structural scaffold and a compositional reservoir. Selectively, B‐site transition metal cations exsolve to form uniformly dispersed high‐entropy alloy (HEA) nanoparticles, whereas A‐site rare‐earth cations remain within the parent framework and transform into a high‐entropy oxide (HEO) support. This unique biphasic high‐entropy heterointerface effectively modulates the interfacial electronic structure and magnetic configuration, which not only enhances polarization loss via abundant heterogeneous interfaces and crystal defects, but also amplifies magnetic loss. The resultant HEA–HEO exhibits an impressive electromagnetic response, with its optimal effective absorption bandwidth showing a 176% and 242% enhancement over low‐entropy materials (La 2 O 3 ‐Co) and single‐phase high‐entropy materials (La 2 O 3 ‐HEA), respectively. In flexible composite films, the heterojunction promotes efficient conversion of electromagnetic energy into heat while enhancing thermal conductivity, thereby broadening pathways towards multifunctional high‐entropy heterojunction materials.