A blocking device for an electromechanical service brake of a motor vehicle, wherein the electromechanical service brake comprises a ratchet gear, supported rotatably about a rotational axis, which ratchet gear is connected or is connectable torque-proof with a drive shaft, rotatable about the same rotational axis of a power train of the electromechanical service brake, and the blocking device comprises a blocking tappet which is axially movable along a displacement axis between a first position, in which the blocking tappet releases the ratchet gear, and a second position, in which the blocking tappet for the purpose of blocking engages into the ratchet gear, a moving system for moving the blocking tappet along the displacement axis, and a position determination system for determining the position of the blocking tappet and an electromechanical service brake for a motor vehicle with such a blocking device and to a motor vehicle with such an electromechanical service brake.