Composite proton-conducting membranes based on perfluorinated short side chain polymers modified with nanodiamonds having a hydrophilic surface owing to grafted carboxyl groups have been synthesized. Membrane films are prepared by precipitating the polymer from a dimethylformamide solution with introduced nanodiamonds. The proposed method for synthesizing the composites provides a high mechanical strength and an increase in the proton conductivity of the material with an increase in temperature in a 20–50°С range at fairly low diamond concentrations (0.25–1 wt %). These features are important for using the composites in fuel cells at high temperatures.