<p>Lightweight polymer composites are attractive for weight-sensitive structural applications; however, their low glass transition temperatures (T_g) often lead to mechanical and dimensional instability at elevated temperature, primarily due to the inherent mobility of polymer chains. Increasing the T_g toward the decomposition temperature is widely accepted strategy to enhance thermomechanical stability. However, increasing chemical crosslink density alone often proves insufficient to suppress segmental motion of polymer under such conditions, especially beyond T_g. Here, we embed rigid three-dimensional nanotube nanocages into a polymer network, yielding an interpenetrated architecture that physically restricts polymer chain mobility by acting as nanoscale structural barriers. This architecture increases the T_g of the resulting nanocomposite to 350&#xa0;°C, compared to 160&#xa0;°C for the neat polymer (~ 119% increase). It also results in a low coefficient of thermal expansion (<InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\:\approx\:\)</EquationSource> </InlineEquation>10 ppm °C⁻¹ at 300&#xa0;°C) and excellent flame retardancy (~ 98% reduction in peak heat release rate). By integrating nanocages with carbon-fiber fabric, the hybrid laminates maintain &gt; 90% modulus retention up to 370&#xa0;°C, exceeding practical titanium alloys, demonstrating remarkable potential for high-temperature aerospace applications.</p>