The present invention relates to a heat pump system for a vehicle which includes a refrigerant-coolant heat exchanger mounted on a refrigerant circulation line of an inlet side of an external heat exchanger to exchange heat between coolant induced into the external heat exchanger and coolant circulating through electronic units of the vehicle, thereby enhancing cooling performance and reducing power consumption without an increase in size of the external heat exchanger by increasing a condensation amount because refrigerant radiates heat while passing through the refrigerant-coolant heat exchanger and the external heat exchanger in an air-conditioning mode and enhancing heating performance and reducing power consumption by increasing an evaporation amount because refrigerant absorbs heat while passing through the refrigerant-coolant heat exchanger, the external heat exchanger and a chiller in a heat pump mode, and increasing temperature of the refrigerant induced into the external heat exchanger to delay frosting of the external heat exchanger.