An outboard motor engine speed control system includes two outboard motors, sensors which detect a travel speed of the boat and the engine speeds of the outboard motors, and a controller which controls the engine speeds of the outboard motors to be synchronized with a highest one of the detected engine speeds when the boat travel speed is equal to or higher than a predetermined value, while controlling the engine speeds of the outboard motors to be synchronized with a lowest one of the detected engine speeds when the boat travel speed is lower the predetermined value. The operations involved in engine speed control of the outboard motors mounted on the boat are thereby simplified and the feel of operation is enhanced.