Solar hydrogen generation is one of the most compelling concepts in modern research to address both the energy and environmental issues simultaneously for the survival of the human race. A Type II heterojunction (CoO-GCN) was fabricated by decorating sub 10 nm CoO nanoparticles (NPs) on the graphitic carbon nitride (GCN) surface. It exhibited improved absorption of UV-VIS light and efficiently separate the photogenerated electrons and holes in opposite directions. A maximum hydrogen generation rate of 9.8 mmol g(-1) h(-1) was recorded using CoO-GCN from 10% aqueous triethanolamine under simulated sunlight in the presence of 1 wt% Pt. The rate is 3.8 times higher than that of bare GCN. Furthermore, it showed excellent stability for up to five repeated uses. Interestingly, the study also revealed that untreated seawater could replace the deionized water. The cooperative participation of the uniform shape and size of CoO NPs firmly grafted on GCN resulted in remarkable performance for solar hydrogen generation.