AbstractMagnetic insulators have wide-ranging applications, including microwave devices, permanent magnets and future spintronic devices. However, the record Curie temperature (T_C), which determines the temperature range in which any ferri/ferromagnetic system remains stable, has stood still for over eight decades. Here we report that a highly B-site ordered cubic double-perovskite insulator, Sr₃OsO₆, has the highest T_C (of ~1060 K) among all insulators and oxides; also, this is the highest magnetic ordering temperature in any compound without 3d transition elements. The cubic B-site ordering is confirmed by atomic-resolution scanning transmission electron microscopy. The electronic structure calculations elucidate a ferromagnetic insulating state with J_eff = 3/2 driven by the large spin-orbit coupling of Os⁶⁺ 5d² orbitals. Moreover, the Sr₃OsO₆ films are epitaxially grown on SrTiO₃ substrates, suggesting that they are compatible with device fabrication processes and thus promising for spintronic applications.