Five samples of Ag-In alloy films with different compositions were deposited on Si substrates by magnetron sputtering. The optical properties of these films were studied by spectroscopic ellipsometry. The dielectric functions of the alloy films significantly increase as the In concentration in the Ag-In alloy films is increased. For Ag-based alloys, a typical interband transition occurs near 3.9 eV. In the tested alloy films, however, this transition energy exhibits a blue shift with increasing In concentration. The optical properties of the alloy films were found to be tunable by varying their In concentrations. The values of the Quality (Q) factors of the Ag0.93In0.07 film are higher than those of other alloy films. At a certain wavelength range, the alloys’ Q factors even exceed those of pure Au and Cu. Our results show that Ag-In alloys have significant potential for application in metamaterials and plasmonic devices.