There is a critical need to develop low cost, metal-free, highly efficient and durable catalysts for industrially important oxygen evolution reactions. Inspired by natural geodes, unique melamine nanogeodes are successfully synthesized using well established hydrothermal process. Sulfur-modified graphitic carbon nitride (S-modified g-CNx) electrocatalysts are obtained by annealing these melamine nanogeodes in situ with sulfur. As compared to the previously reported non-metallic systems and well-established metallic catalysts, the S-modified g-CNx nanostructures achieved superior performance, requiring a lower overpotential (290 mV) to achieve a current density of 10 mA cm-2 and a Tafel slope of 120 mV dec-1 with long-term durability of 91.2% retention for 18 h. These low-cost, ecologically friendly, and easy-tosynthesize catalysts with unusual performance will have a great influence in the field of oxygen evolution reaction electrocatalysis.