A heat pump water heater appliance includes an electronic mixing valve. The electronic mixing valve is positioned within a casing of the heat pump water heater appliance. A mixed water conduit and a heated water conduit of the electronic mixing valve may be positioned coaxially with each other, and a cold water conduit of the electronic mixing valve may be oriented such that the cold water conduit extends orthogonal to the heated water conduit and the mixed water conduit. A plunger is positioned within the valve body at the mixing chamber of the valve body. A motor is coupled to the plunger, the motor being operable to adjust a position of a head of the plunger within the valve body.