A control device and a transmission system are configured to improve riding comfort of a human-powered vehicle. The control device includes an electronic controller that is configured to control a transmission device of a human-powered vehicle so that a transmission ratio at which the vehicle is started becomes equal to a designated transmission ratio. The electronic controller is configured to set the designated transmission ratio based on reference information that excludes information related to a gradient of a road surface, information related to manual operation of the transmission device, and information related to propulsion assistance of the human-powered vehicle.