A valve actuator includes a housing, a motor disposed on the housing, a driving gear coupled to a motor shaft of the motor, a transmission gear which is in external contact with the driving gear and rotates according to a predetermined gear ratio when the driving gear rotates, an output shaft including a main body and a plurality of protrusions radially protruding from the main body and spaced apart from each other in a circumferential direction, an output gear coupled to the output shaft and in external contact with the transmission gear, a stopper disposed in the housing and limiting a rotation radius of the output gear, and an elastic member disposed between a lower portion of the plurality of protrusions and a lower plate of the housing.