An information processing system includes a vehicle, a terminal device, and a server that can communicate with the vehicle and the terminal device. The vehicle repeatedly transmits positional information on the vehicle to the server at predetermined timings. The terminal device repeatedly transmits user identification information for identifying a user of the terminal device and positional information on the terminal device to the server at predetermined timings. The server determines, based on the positional information on the vehicle and the positional information on the terminal device, whether or not the terminal device is located in a reference region within a predetermined distance from the vehicle, and determines, based on the user identification information, that the user of the terminal device is on board the vehicle, when it is determined that the terminal device is located in the reference region.