An inorganic–organic composite membrane composed of Fe0.4Ta0.5P2O7 (FTPO) and sulfonated polystyrene-b-poly(ethylene/butylene)-b-polystyrene (sSEBS) is prepared and characterized. To gain satisfaction of both the proton conductivity and mechanical strength in composite membranes, the optimal content of sSEBS was determined to be 30wt%, which resulted in high proton conductivities of approximately 0.01Scm−1 between 50 and 150°C even under an unhumidified condition, a tensile strength of 4.1MPa, and an elongation at break of 613%. A homogeneous distribution of the FTPO particles in the matrix was achieved at the composite membrane thickness greater than 50μm, providing gas-impermeability and low area-specific resistance (ASR) of the membrane (0.17Ωcm2 at 150°C and H2O vapor concentration of 30.8vol%). Fuel cell tests using the composite membrane provided peak power densities of 265 and 303mWcm−2 at 150°C under an unhumidified condition and at a H2O vapor concentration of 30.8vol%, respectively.