An in-vehicle temperature adjustment system includes: a refrigeration circuit including an inter-medium heat exchanger and a vaporizer that vaporizes the cooling medium to achieve a refrigeration cycle by circulating a cooling medium; a thermal circuit including a heater core, the inter-medium heat exchanger, an engine thermal circuit, and a radiator to circulate the heating medium; and a controller that controls a distribution state of the heating medium. The thermal circuit includes: a first branch portion at which a coolant flowing out of the engine thermal circuit and the inter-medium heat exchanger is divided into coolants flowing into the heater core and into the radiator; a second branch portion at which a coolant flowing out of the heater core is divided into coolants flowing into the inter-medium heat exchanger and into the engine thermal circuit; a first adjustment valve and a second adjustment valve.