Improving the performance of thermoelectric (TE) materials that can convert waste heat into electricity is becoming increasingly important. Accordingly, the dimensionless figure of merit, ZT needs to be increased. Al, a group-III element, has not been widely used as a dopant in GeTe-based thermoelectric materials because it acts as a p-type dopant and thus further increases the hole carrier concentration of GeTe, which has already a high intrinsic hole concentration due to inherent Ge vacancies. Therefore, Al as a dopant in GeTe has been considered to have detrimental effects on the resulting thermoelectric performance. In this study, we aim to enhance the suitability of Al doping in GeTe-based materials through synergistically co-doping it with Bi. In this case, co-doping Al and Bi optimizes the carrier concentration and induces a high Seebeck coefficient from an additional increased effective mass and low total thermal conductivity arising from the numerous phonons scattering centers and decreased electronic contribution to heat transport. Consequently, a maximum ZT of ∼1.95 at 673 K with an increased average ZT is achieved. These results indicate that the p-type dopant Al, co-doped with Bi in GeTe-based thermoelectric materials is one of the very effective dopants which can eventually improve the overall TE performance of these materials for useful waste heat recovery application and cooling.