Provided is a transmission control system that enables more precise control of the transmission ratio. This transmission control system is provided with a motor, an actuator, a transmission, a position sensor, a current sensor, and a control unit. The control unit includes: a drive unit that drives the motor while the vehicle is stopped; and a first estimation unit that estimates the initial transmission ratio, which indicates the transmission ratio when the ignition switch is turned on, on the basis of a rotation position detected by the position sensor, a load current detected by the current sensor, and a characteristic curve over a period during which the drive unit is operating.