An embodiment heat pump system for a vehicle includes an air conditioner unit and a chiller. The air conditioner unit includes a compressor, first through third heat-exchangers, a first expansion valve, and an evaporator interconnected by a refrigerant line for circulating a refrigerant through the refrigerant line, and the chiller is connected to the refrigerant line through a first connection line for adjusting a temperature of a coolant by heat-exchanging the refrigerant with the coolant. The air conditioner unit further includes a second expansion valve on the first connection line upstream of the chiller, a first valve on the refrigerant line between the first and second heat-exchangers, a third expansion valve on the refrigerant line between the first valve and the second heat-exchanger, a second valve on the refrigerant line between the evaporator and the compressor, and a fourth expansion valve on a second connection line.