Recent progress in portable and wearable electronics has promoted a growing demand for highperformance and flexible energy-storage devices that are abundant and affordable. Because reduced graphene oxide (rGO), originating from inexpensive graphite, serves as higher-performance energystorage electrode than conventional activated carbons and carbon nanotubes, research and development of rGO/polymer composite electrodes for flexible supercapacitors have become a center of attraction.However, the fabrication of rGO-based flexible electrodes frequently requires long time with hightemperature treatment or toxic chemical treatment, resulting in the lack of scalability and ecofriendliness. Here we show fast, scalable, and environment-compatible route to fabricate highperformance rGO/cellulose paper supercapacitor electrode. Single-layer graphene oxide (GO) sheets and recycled waste pulp fibers were successfully fabricated into a paper composite by a wellestablished scalable papermaking process, followed by a room-temperature, additive-free, and millisecond-timescale flash reduction process. The as-prepared rGO/paper electrode had high specific capacitance, up to 212 F g -1 , for an all paper-based flexible supercapacitor, comparable to those of state-of-the-art rGO-based electrodes, while dramatically decreasing the reduction time of GO from the conventional hour timescale to milliseconds. This work will pave the way for green, flexible, and massproducible energy-storage papers in future wearable electronics. IntroductionSince the invention of paper approximately 2000 years ago, it has been traditionally fabricated from an aqueous suspension of cellulose fibers by papermaking, which is a sequential process of filtration, dewatering, and drying. The current papermaking process enables high-speed and large-area fabrication of paper materials up to 18,000 m 2 min -1 . Therefore, paper materials are mass-producible and inexpensive, and they have been used for many purposes, such as writing, printing, and packaging applications, in daily life. Recently, the application of paper has been extended to electronics; [1,2] electronic devices have been fabricated on paper substrates, including inorganic [3,4] and organic transistors, [5] complementary metal oxide semiconductors, [6,7] triboelectric nanogenerators, [8] memory, [9,10] transparent conductive films, [11,12] and antennas. [13,14] Paper electronics provides new possibilities for next-generation devices with flexible, biodegradable, biocompatible, and eco-friendly electronics.Recent years have seen remarkable progress in wearable electronics; [15,16] portable consumer electronics are becoming flexible, lightweight, and even wearable. Because wearable electronics essentially require energy-storage devices with thin, lightweight, flexible, and conformable properties, such flexible energy-storage devices have become the focus of major research. [17][18][19] Of various energystorage devices, supercapacitors have attracted much attention because of their high power den...