Polydispersed CoP nanoparticles in an orthorhombic phase were synthesized via a gas-solid reaction and then deposited over graphitic carbon nitride to build the CoP/g-C3N4 (CoP-CN) heterostructure. Nanorod-like CoP nanoparticles with a length of 10-80 nm were connected to g-C3N4 nanosheets to build an intimate face-to-face contact via their crystal planes of (011) and (211). This unique heterojunction hybrid exhibits superior photocatalytic and photoelectrochemical performances for H-2 evolution and photoelectrochemical response plus excellent overall water-splitting activity. The optimal sample of 3% CoP-CN composite achieved a superior hydrogen production rate at 1038.1 mu mol h(-1) g(-1) when irradiated by simulated solar light, exhibiting a much higher photocurrent at 150 mu A cm(-2) compared to pure g-C3N4. Also, a larger anodic current density was detected during the photoelectrochemical hydrogen evolution reactions (PEC HERs) with enhanced applied bias photon-to-current efficiency, denoting a higher efficiency for PEC HER The enhancements for photocatalytic and PEC HER activity are mainly attributed to the formation of intimate interfacial contact for better light absorption, stronger photoreductive potentials, and higher efficiency for charge separation and transfer. This study provides a proof-of-concept design and construction of effective cobalt-phosphide-based heterojunctions for hydrogen evolution and water-splitting applications.