Cotton fibers, a natural cellulose, have played a critical role in the development of wearable energy storage, owning to their wearability, integrability, eco-benignity, and cost effectiveness. Graphene, a two-dimensional carbon material, possesses excellent electrochemical properties that can be incorporated into cotton yarns to enhance their performance. However, the conventional chemical reduction processes use corrosive chemical reducing agents that limit their wide application. In the current study, a reduced graphene oxide (rGO)/cotton (RC) yarn supercapacitor was fabricated employing a facile and green hydrothermal approach without the use of any chemical reductants. The reduction efficiency of external GO nanoparticles and the morphology change of internal cotton fiber under different hydrothermal treatments are investigated to explore their effects on the energy storage performance of the resultant RC electrodes and supercapacitors. The RC yarn electrode can reach a capacitance of 13.31 mF cm-1 at of 0.1 mA cm-1. The assembled asymmetric supercapacitor exhibited a high specific capacitance of 2.99 mF cm-1 (9.54 mF cm-2, and 381.43 mF cm-3) at 0.02 mA cm-1 (0.06 mA cm-2 and 2.55 mA cm-3) with a high energy density of 0.42 mu Wh cm-1 (1.32 mu Wh cm-2 and 0.05 mWh cm-3) at a power density of 10 mu W cm-1 (31.84 mu W cm-2 and 1.27 mW cm-3). This work develops a green versatile strategy for fabricating RC supercapacitors, which may provide a new avenue for wearable energy storage devices.