A transmission control system is configured to be used with a human-powered vehicle that includes a crank, a first rotation body rotated independently from the crank, a drive wheel, a second rotation body rotated independently from the drive wheel, a transfer body that transfers rotation force between the first and second rotation bodies, and a transmission that controls the transfer body and shifts a transmission ratio. The transmission control system includes a motor that drives the transfer body, a first detector that detects at least one of acceleration and vibration of the human-powered vehicle, and an electronic controller configured to control the motor in accordance with a detection result of the first detector upon generation of a shift request for the transmission in a state in which the drive wheel is rotated and a rotational angle of the crank is maintained in a predetermined range.