Graphitic carbon nitride nanosheets are extracted, produced via simple liquid-phase exfoliation of a layered bulk material, g-C3N4. The resulting nanosheets, having approximate to 2 nm thickness and N/C atomic ratio of 1.31, show an optical bandgap of 2.65 eV. The carbon nitride nanosheets are demonstrated to exhibit excellent photocatalytic activity for hydrogen evolution under visible light.