Sulfur-functionalized graphene monoliths with a high sulfur fraction (16.8 wt%) were prepared to demonstrate a high capacity (∼400 mA h g-1 at 0.1 A g-1) and ultrafast (∼120 mA h g-1 at 5 A g-1) sodium ion storage. The reversible reaction of -C-Sx-C- with sodium ions contributes to the extra capacity while a 3D graphene network guarantees high rate capability.