A tire position determination system includes a position determination unit that determines a tire position by obtaining axle rotation information when each of tire pressure transmitters reaches a specific position on a tire rotation path of a corresponding tire and specifying a tire that rotates in synchronism with the axle rotation information of each axle. A cycle calculator calculates a rotation cycle of each axle based on the axle rotation information. A validity determination unit determines validity of accuracy of gravity sampling based on the rotation cycle, a gravity sampling interval time of a gravitational component acting on each tire pressure transmitter, and a gravity sampling frequency in the rotation cycle. The position determination unit determines the tire position based on a determination result of the validity determination unit.