The present invention relates to a heat pump system for a vehicle. The heat pump system includes a refrigerant-coolant heat exchanger for exchanging heat between refrigerant circulating through a refrigerant circulation line and coolant circulating through electronic units of the vehicle. The heat pump system can operate the heat pump mode even though outdoor air temperature is zero or lower or frosting is formed on the external heat exchanger because waste heat of the electronic units is retrieved through the refrigerant-coolant heat exchanger, thereby further enhancing heating performance and efficiency.