Polymer dielectrics with high breakdown strength (E_b) and high efficiency are urgently demanded in advanced electrical and electronic systems, yet their energy density (U_e) is limited due to low dielectric constant (ε_r) and high loss at elevated temperatures. Conventional inorganic fillers with diameters from nano to micrometers can only increase ε_r at the cost of compromised E_b and U_e due to their poor compatibility with polymer matrix. Herein, hydroxyapatite (HAP) sub‐nanowires with a diameter of ≈0.9 nm are incorporated in polyetherimide (PEI) matrix to form HAP/PEI sub‐nanocomposites. ε_r and E_b of the composites are concomitantly enhanced with only 0.5 wt.% of HAP sub‐nanowires, leading to high U_e of 5.14 (@150 °C) and 3.1 J cm⁻³ (@200 °C) with efficiency of 90% and high‐temperature stability up to 3 × 10⁵ charge‐discharge cycles at 200 °C. Microstructural analysis and molecular dynamics simulations indicate that the sub‐nanowires with comparable diameter as polymer chains induce enormous interfacial area, substantially increase mobility of polymer chains and form dense traps for charge carriers. This work extends the current research scope of polymer‐inorganics composite dielectrics to the sub‐nano‐level incorporation and provides a novel strategy for fabricating high performance polymer dielectrics at elevated temperatures.