A vehicle power supply system, being mounted on a vehicle, includes: a main power supply system including a main low-voltage power supply and a normal load; and a backup power supply system including a backup low-voltage power supply and an emergency important load and connected to the main power supply system. A backup power supply control device of the backup power supply system is configured to execute a backup low-voltage power supply state estimation processing, and output a signal indicating that the backup low-voltage power supply is in a state allowing supplying electric power for operating the emergency important load based on an estimation result of the backup low-voltage power supply state estimation processing, in a case in which the state of the vehicle does not satisfy a predetermined condition.