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Abstract The realization of low thermal conductivity at high temperatures (0.11 W m⁻¹K⁻¹800 °C) in ambient air in a porous solid thermal insulation material, using stable packed nanoparticles of high‐entropy spinel oxide with 8 cations (HESO‐8 NPs) with a relatively high packing density of ≈50%, is reported. The high‐density HESO‐8 NP pellets possess around 1000‐fold lower thermal diffusivity than that of air, resulting in much slower heat propagation when subjected to a transient heat flux. The low thermal conductivity and diffusivity are realized by suppressing all three modes of heat transfer, namely solid conduction, gas conduction, and thermal radiation, via stable nanoconstriction and infrared‐absorbing nature of the HESO‐8 NPs, which are enabled by remarkable microstructural stability against coarsening at high temperatures due to the high entropy. This work can elucidate the design of the next‐generation high‐temperature thermal insulation materials using high‐entropy ceramic nanostructures.