The vehicle power supply system includes the DC/DC converter that is disposed on an electric path between the first load related to vehicle control and the battery and converts a voltage of direct current power supplied from the battery to the first load, and the detection device that detects the voltage of the battery. In a case where the engine is restarted by using the starter from the automatically stopped state of the engine, the detection device controls the DC/DC converter to start in a case where the voltage of the battery becomes equal to or less than a threshold value during a period from stoppage of the alternator to start of the starter.