In the present paper, we report a novel ion-exchange carbon nanomaterial, surface sulfonated carbon nanofiber (S-CNF) fabrics, with a high surface area and high ion-exchange and adsorption capacities. S-CNF fabrics bearing SO3H groups were prepared by electrospinning, successive carbonization and sulfonation. We observed a nanosize effect, in which the surface area, and ion-exchange and adsorption capacities significantly increase with a decrease in the fiber diameter. The prepared fabrics with a diameter of 80 nm had the maximum values for the total pore surface area of 32.2 m2 g−1, the ion-exchange capacity of 2.94 mmol g−1, and an adsorption capacity of 943.1 mg g−1 for methylene blue (MB). The MB adsorption capacity is two times higher than that of commercial activated carbons. These results highlight the fact that the surface functionalized CNF fabrics with ionic groups are promising materials for high-capacity ion-exchangers.