To provide a rear wheel fender structure for motorcycles, which can be molded with a simplified and inexpensive mold assembly even where the rear wheel finder is so designed and so structured as to permit component parts to be fitted thereto and as to block the splashing of water and/or mud, the rear wheel fender structure 27 is positioned above the motorcycle rear wheel 14 and includes an upper fender 39 and a lower fender 40 secured to a bottom region of the upper fender 39. An adiabatic space 99 is preferably formed between the upper fender 39 and the lower fender 40.