A straddled-vehicle authentication system is provided that achieves less power consumption while maintaining security, and also achieves a suppression of an increase in the size of the straddled vehicle which may result from an increase in the size of a battery. A straddled-vehicle authentication system includes: a power unit control unit that controls a power unit, the power unit control unit provided in the straddled vehicle, the power unit provided in the straddled vehicle and configured to drive the straddled vehicle; a portable device that is used for an authentication to use the straddled vehicle via wireless communication, the portable device being physically separate from the straddled vehicle; an authentication control unit that performs the authentication to use the straddled vehicle by communicating with the portable device, the authentication control unit provided in the straddled vehicle; a main operation part that receives an operation for switching the state of the authentication system to any of an OFF state, an authentication verified state, and a power-unit ON state, the main operation part provided in the straddled vehicle, the OFF state being a state in which a power source of the power unit control unit is off and the authentication via a communication between the authentication control unit and the portable device is not performed, the authentication verified state being a state in which the power source of the power unit control unit is off and the authentication via a communication between the authentication control unit and the portable device has been performed, the power-unit ON state being a state in which the power source of the power unit control unit is on and the authentication via a communication between the authentication control unit and the portable device has been performed; and an unauthorized access prevention unit that performs a process for preventing an unauthorized access to the straddled vehicle in accordance with at least either one of an elapsed time in the authentication verified state and a status of authentication performed by the authentication control unit in the authentication verified state.