A vehicle power supply system includes: a main power supply system provided with a main low-voltage power supply; and a backup power supply system provided with a backup low-voltage power supply. The backup power supply system includes a backup power supply controller. When the vehicle power supply system is turned off and a state of the vehicle satisfies a predetermined condition, the backup power supply controller executes a backup low-voltage power supply state estimation processing of estimating whether the backup low-voltage power supply is in a state in which the backup low-voltage power supply is able to supply electric power for operating the emergency important load.