The growing demand for high-speed electronic communication devices has highlighted the need for advanced low-dielectric insulating materials. Herein, we present a systematic design strategy for low-k poly(aryl ether ketone) (PAEK) materials from molecular-level engineering to crosslinked polymer network design. PAEKs incorporating cyclohexane units with various methyl substitutions were synthesized to tailor the free volume and chain mobility. To further optimize the dielectric properties and mechanical robustness, crosslinkable vinyl groups were introduced into the polymer backbone and subsequently crosslinked using octavinyl-POSS nanoparticles. The resulting polymer–POSS composite exhibit a low dielectric constant of 2.53 with a dielectric loss of 0.0027 at 10 GHz, together with enhanced thermal and mechanical stability. This molecular- and network-level design strategy provides valuable insights into the development of next-generation high-performance low-k polymer substrates.