Three-dimensional graphene foam (3DGF) with high conductivity and rich porosity is an ideal carbon-based metal-free electrocatalyst for hydrogen evolution reaction (HER); however, its HER performance is still very poor due to insufficient electrocatalytic active sites. Herein, in order to create many more electrocatalytic active sites, a facile approach combining Ar plasma pretreatment and subsequent N, S co-doping is presented. As a freestanding and binder-free electrocatalyst, the Ar-plasma-pretreated and N, S co-doped 3DGF (3DGF-Ar-NS) demonstrates significantly enhanced HER performance: the Tafel slope of 3DGF-Ar-NS dramatically decreases from 182 mV/dec (3DGF) to 75 mV/dec. The excellent HER performance and long-term stability of 3DGF-Ar-NS can be attributed to the synergistic effect of the 3DGF skeleton, Ar plasma pretreatment and heteroatom doping: the conductive, porous and flexible 3D graphene skeleton provides the interconnected conductive paths and facilitates the charge transportation; the Ar plasma pretreatment and N, S co-doping effectively create many more electrocatalytic active sites, resulting in significant enhancement in HER performance. This work provides a facile strategy to tailor and enhance the electrocatalytic performances of carbon-based materials for hydrogen evolution applications. (C) 2017 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.