In this study, Pb18Sb5Te20 is used as a substrate to which different amounts of Ag are added to form Ag1−yPb18Sb5Te20 (y = 0, 0.1, 0.2, 0.3, 0.4) alloys. All the experiments use vacuum arc melting to produce a series of bulk thermoelectric materials. X-ray diffraction (XRD) analysis and Joint Committee on Powder Diffraction Standards (JCPDS) database are used to confirm the crystal structure of all materials. This study investigates the thermoelectric properties of the alloys using the Seebeck coefficient, thermal conductivity, and resistivity parameters. The addition of Ag enhances the thermoelectric figure of merit (ZT) value of PbSbTe-based thermoelectric materials. Scanning electron microscopy (SEM) was also used to observe the fracture surface of all tested materials.