A seat for a vehicle in which, when the vehicle has a side collision, pressing against a side support portion of a seat cushion by a vehicle body side portion and deformation of the side support portion to the upper side may be suppressed, and in which excellent seating comfort of the seat cushion may be maintained. At a rear seat, when a door trim (32) of a rear side door (30) collides with a side support portion (16A) due to an impact load F of a side collision, a dilatant member (46) provided at the side support portion (16A) hardens instantly. As a result, deformation of the side support portion (16A) is suppressed, and deformation of the side support portion (16A) to the upper side may be suppressed. Therefore, a waist area chamber (28B) of a rear side airbag (28) may be caused to deploy between the waist area of a seat occupant (14) and the door trim (32) as intended.