A heat pump system for a vehicle includes a dehumidification line for supplying some refrigerant circulating in a refrigerant circulation line to an evaporator before the refrigerant is introduced into an exterior heat exchanger after passing a first expansion means so as to dehumidify the interior of the vehicle in a heat pump mode, thereby allowing the refrigerant to smoothly flow to the evaporator at a low pressure through the dehumidification line before the refrigerant is introduced into the exterior heat exchanger which has a higher pressure than the evaporator when the interior of the vehicle is dehumidified, and smoothly dehumidifying the inside of the vehicle.