In this work, a range of g-C3N4/MnO composites were constructed using g-C3N4 nanosheets modified with MnO, and the photocatalytic performance for hydrogen evolution was evaluated by using these as-prepared g-C3N4/MnO composites as photocatalysts. It was found that the photocatalytic activity of the g-C3N4/MnO composites for hydrogen evolution is significantly enhanced compared with that of pristine g-C3N4 since the formation of heterojunctions between the MnO nanoparticles and g-C3N4 nanosheets through coordination covalent bonds promotes the charge carrier transfer and separation abilities of the composites. The loading mass of MnO also has a large influence on the photocatalytic activity of the g-C3N4/MnO composites. Particularly, the g-C3N4/MnO-5 composite with 5 wt% MnO shows superior photocatalytic activity with a hydrogen evolution rate of 559 μmol h-1 g-1 under visible light, which is about 9 times that of the bulky g-C3N4. These findings demonstrate that the combination of metal oxides and g-C3N4 to construct composite photocatalysts is an effective method to improve the photocatalytic performance.