A side table assembly for a vehicle seat includes a table on which a cup holder inserting part is formed, a body arranged on a side face of the vehicle seat, a joint hinge device coupled between the body and the table to allow the table to be rotated with respect to the body; and a cup holder having an opened upper face and a closed lower face and coupled to the cup holder inserting part. The joint hinge device includes a joint hinge connected to the table at one side and connected to the body at another side, a hinge spring inserted into the joint hinge to elastically support the joint hinge, and a hinge cylinder having one side inserted into the joint hinge and being in contact with the hinge spring and another side inserted into by the body.