Materials　that　can　provide　reliable　electromagnetic　interference　(EMI)　shielding　in　highly　oxidative　atmosphere　at　elevated　temperature　are　indispensable　in　the　fast-developing　aerospace　field.　However,　most　of　conductor-type　EMI　shielding　materials　such　as　metals　can　hardly　withstand　the　high-temperature　oxidation,　while　the　conventional　dielectric-type　materials　cannot　offer　sufficient　shielding　efficiency　in　gigahertz　(GHz)　frequencies.　Here,　a　highly　deficient　medium-entropy　(ME)　perovskite　ceramic　as　an　efficient　EMI　shielding　material　in　harsh　environment,　is　demonstrated.　The　synergistic　effect　of　entropy　stabilization　and　aliovalent　substitution　on　A-site　generate　abnormally　high　concentration　of　Ti　and　O　vacancies　that　are　stable　under　high-temperature　oxidation.　Due　to　the　clustering　of　vacancies,　the　highly　deficient　perovskite　ceramic　exhibits　giant　complex　permittivity　and　polarization　loss　in　GHz,　leading　to　the　specific　EMI　shielding　effectiveness　above　30 dB/mm　in　X-band　even　after　100　h　of　annealing　at　1000 °C　in　air.　Along　with　the　low　thermal　conductivity,　the　aliovalent　ME　perovskite　can　serve　as　a　bifunctional　shielding　material　for　applications　in　aircraft　engines　and　reusable　rockets.　©　2024　Wiley-VCH　GmbH.