A transmission control system is provided for used in a bicycle having a crank, a front sprocket that is rotatable independent of the crank, a rear wheel, a rear sprocket that is rotatable independent of the rear wheel, a chain that engages with the front sprocket and the rear sprocket, and at least one external transmission mechanism. The transmission control system is basically provided with a driving force output device, a detection device and at least one controller. The detection device is one of a rotational state detection device, a manual drive force detecting device and a speed detection device. The at least one controller is programmed to control the driving force output device to drive the chain and to control a gear shifting action of the at least one external transmission mechanism to shift gears, based on a comparison of the detection results by the detection device and a prescribed state or a prescribed value.