Composite Ormosil/Nafion membranes were prepared and characterized for use as electrolytes in direct methanol fuel cells (DMFCs). An organosilane derivative (sulfonated diphenylsilanediol, SDPSD) was selected as a filler of the Nafion matrix. The composite membranes were characterized by electrochemical impedance spectroscopy, differential scanning calorimetry, and solvent uptake measurements. The composite membranes exhibited higher proton conductivity and enhanced stability than the reference unfilled Nafion membrane, due to the occurrence of an effective interaction between the filler and the polar cluster of the polymer matrix. Polarization curves in a DMFC were acquired and the results showed that the performance of the composite membrane was superior to that of unfilled Nafion due to a reduced methanol permeation rate, as well as to enhanced proton conductivity and thermal stability of the membrane. Due to its satisfactory properties, the composite Nafion/SDPSD membrane has a potential use as electrolyte in DMFCs operating at intermediate temperatures.