A cup holding device for a foldable seat includes a connecting part and a cup holder. The connecting part is provided on an armrest frame or a front leg frame of a foldable seat, and is located below a seat portion of the foldable seat. The cup holder is mounted on the connecting part, and has a receiving cavity adapted to receive the placement of a container. By placing the connecting part and the cup holder below the seat portion of the foldable seat, the present invention can make use of the space at the bottom of the foldable seat and prevent a drinking container from being accidentally hit by a person's swinging arm causing its tipping over.