Indium was substituted at gallium site in chalcopyrite AgGaS2 structure by using a simple solid solution method. The spectroscopic analysis using extended x-ray absorption fine structure and x-ray photoelectron spectroscopy confirmed the indium substitution in AgGaS2 lattice. The band gap energy of AgGa1−xInxS2 (x=0–1) estimated from the onset of absorption edge was found to be reduced from 2.67eV (x=0) to 1.9eV (x=1) by indium substitution. The theoretical and experimental studies showed that the indium s orbitals in AgGa1−xInxS2 tailored the band gap energy, thereby modified the photocatalytic activity of the AgGa1−xInxS2.