A one-step synthesis method is introduced and used to form an ultrathin, homogeneous organic–inorganic hybrid dielectric film with a high dielectric constant (high-k), based on initiated chemical vapor deposition. The hybrid dielectric is synthesized from tetrakis-dimethyl-amino-zirconium and 2-hydroxyethyl methacrylate, which are a high-k inorganic material and a soft organic material, respectively. A detailed material analysis on the synthesized ZrOx-organic hybrid (Zr-hybrid) is performed. The Zr-hybrid dielectric has a high dielectric constant of nine, leading to a film equivalent oxide thickness (EOT) as low as 3.2 nm, which is the lowest EOT obtained from a flexible dielectric layer to date. The leakage current density (J) is no larger than 6 × 10–7 A/cm2 at 2 MV/cm, and the breakdown field (Ebreak) was ∼3.3 MV/cm. The J of the Zr-hybrid dielectric remains almost constant even under the 2.5% strain condition, while that of the ZrO2 dielectric breaks down electrically at a tensile strain of less than 1.0%. The Zr-hybrid dielectric shows an energy band gap in the range of 5.2–5.4 eV and exhibits a sufficient valence band offset of around 3.0 eV with a pentacene organic semiconductor. The gate stack of the Zr-hybrid dielectric/pentacene semiconductor shows decent metal–oxide–semiconductor field-effect transistor performance even under a tensile strain of 1.67%, indicating that the Zr-hybrid is a promising gate dielectric for advanced flexible electronic applications.