Measurements and modeling of electronic transport properties of n-type Ag2+xSe suggest that this material could have a thermoelectric figure of merit zT greater than 1 at 300 K and above. The exceptional performance can be traced to the exceptionally high mobility, higher than other optimized thermoelectric materials. Although zT decreases at high temperature because of a transition to a phase with high carrier concentration, our model indicates that reducing the carrier concentration will lead to high thermoelectric performance at room temperature for cooling applications as well as up to 600 K for waste heat recovery.