Bio‐based eugenol‐derived polybenzoxazines were synthesized from hexamethylene diamine (Eu‐HMD), isophorone diamine (Eu‐IPD), and norbornane diamine (Eu‐NBD) and reinforced with 5–20 wt. % thiol‐functional polyhedral oligomeric silsesquioxane (SH‐POSS) to develop multifunctional organic–inorganic hybrid composites. The incorporation of SH‐POSS significantly enhanced material performance while preserving high optical transparency (~90%–95% in the visible region). Data from thermal analysis infer that the improvement in thermal stability and increased char yield is due to the presence of a thermally robust SiOSi cage structure and barrier effect offered by the POSS moiety. SEM observations showed smooth, defect‐free surfaces for both neat matrix and hybrid composite coatings before and after UV exposure, while SH‐POSS reinforced systems maintained uniform nanoscale dispersion with slightly more defined cubical domains, confirming structural stability without photodegradation. The hybrid coatings exhibited superior resistance to UV‐induced degradation compared with that of neat polymer matrices. Surface wettability increased markedly with POSS loading, with water contact angles reaching up to 143° at 20 wt. %, indicating enhanced hydrophobicity and moisture resistance. Dielectric measurements yielded the values of reduced dielectric constant (~2.50 at 10 MHz) and dielectric loss (~0.0034 at 10 MHz), attributed to the low polarizability and intrinsic nanoscale free volume of POSS nanocages. Overall, SH‐POSS reinforced bio‐based polybenzoxazines possess thermally stable, UV‐durable, hydrophobic, low‐moisture‐absorbing, and low‐k dielectric properties suitable for advanced protective coatings and microelectronics insulation applications.