An embodiment heat pump system of a vehicle includes a first cooling device including a battery module and a first line for circulating a coolant, an air conditioner unit including a compressor, an internal condenser, a main heat exchanger, a first expansion valve, and an evaporator that are interconnected through a refrigerant line for circulating a refrigerant, and a chiller connected to the first cooling device through the first line, connected to the refrigerant line through a first connection line, and configured to adjust a coolant temperature through heat-exchanging a selectively introduced coolant with the refrigerant. The air conditioner unit further includes a second expansion valve, a first valve, a second valve, a second connection line, a third connection line, and a fourth connection line.