A seatpost has a post body, an adjusting mount, a connecting unit, and a connecting bracket. The post body has a supporting base disposed at an upper end of the post body. The supporting base has a bore formed through a rear end of the supporting base. The adjusting mount is connected to the supporting base and has a passing hole disposed at a rear end of the adjusting mount. The connecting unit is connected to the adjusting mount and has a connecting portion having a threaded hole. A bolt passes through the bore, the passing hole, and is screwed with the threaded hole. The connecting bracket has a front end connected to the connecting portion, a rear end opposite the front end, and an assembling board disposed at the rear end of the connecting bracket for assembling a taillight or a bicycle license plate.