A kappa-carrageenan-Pt nanoparticle composite (Cg-Pt) was synthesized and its proton conductivity was examined by a complex-plane impedance method. The synthesized Cg-Pt was characterized by transmission electron microscope (TEM) observation, powder X-ray diffraction (XRD), Fourier transformed infrared (FT-IR) absorption measurements, and thermogravimetry/mass spectrometry (TG/MS) analysis. It was revealed that the a.c. electrical conductivity of Cg-Pt strongly depends on relative humidity (RH) and exceeds the conductivity of Cg under conditions of high humidity. From the temperature dependence of the a.c. conductivity, activation energies for protonic conduction were estimated to be 0.47 and 0.34 eV for Cg-Pt and Cg, respectively. The origin of the differences in the conductivities and activation energies are discussed.