An embodiment manual seatback folding apparatus of an electric seat for a vehicle includes a shaft fastened to a seatback frame, wherein first and second end portions of the shaft are rotatably inserted into first and second seat cushion side frames, respectively, a first rotatable disk including a first locking recess, a second rotatable disk including a second locking recess and a pusher, a first manual control device detachably engaged with the first locking recess, a second manual control device to be connected to the first manual control device through a cable and detachably engaged with the second locking recess, and a switch device mounted on an outer surface of the second seat cushion side frame and configured to perform a switching operation in response to the pusher in a state in which the second rotatable disk is rotated in a folding direction.