Two types of silver nanoparticle (Ag NP)-dye nanocomposite layers were prepared on an indium-tin oxide (ITO) transparent electrode, where porphyrins were fixed on the Ag NPs by chemical bonding through their carboxylate moieties or by hydrophobic interactions. Stable cathodic photocurrents were generated from the nanocomposite layer structures and were specifically enhanced by the overlap of the Q-band excitation of the porphyrins between 500 and 600 nm with the gap-mode plasmon band of the Ag NPs. The photocurrent efficiency of the chemical bonding system was significantly higher than that from the hydrophobic interaction system.