An armrest assembly for an automobile seat, in which an armrest protrudes from a foldable seatback such that a passenger rests his/her arm on the armrest when the seatback is folded, and comes into contact with the seatback when the seatback is folded out in order to take a seat. The armrest assembly includes the armrest, which is installed on a seatback of the seat via links so as to protrude from the seatback, an external force transmitting device, which is coupled with at least one of the links, and transmits external force to the coupled link such that the armrest protrudes from the seatback, and a pressing unit, which installed on one side of the seatback such that the external force is applied to the external force transmitting device when the seatback is folded.