An embodiment is a swing armrest for a seat, the swing armrest including a fixation bracket fixed to a seatback frame or fixed to a seatback bracket that is fixed to the seatback frame, a rotation bracket rotatably fixed to the fixation bracket by a rotation pin, a return spring configured to elastically support the rotation bracket in a direction in which the rotation bracket rotates with respect to the fixation bracket, a fixing post protruding from the rotation bracket to a side of the seat, and an armrest coupled to the fixing post.