Cu2O has emerged as a promising material for sustainable hydrogen production through photoelectrochemical (PEC) water splitting, while inefficient charge separation remains one of the main challenges hindering its development. In this work, a new architecture of InAlN/Cu2O heterojunction photocathode is demonstrated by combining n-type InAlN and p-type Cu2O to improve the charge separation efficiency, thus enhancing PEC water-splitting performance. The Pt/InAlN/Cu2O photoelectrode exhibits a photocurrent density of 2.54 mA cm(-2) at 0 V versus reversible hydrogen electrode (V-RHE), which is 3.21 times higher than that of Cu2O (0.79 mA cm(-2) at V-RHE). The enhanced PEC performance is explained by the larger built-in potential V-bi of 1.43 V formed at the InAlN/Cu2O p-n junction than that in the single Cu2O photocathode (V-bi < 0.77 V), which improves the separation of the photogenerated carriers and thus relieves the bottlenecks of charge-transfer kinetics at the electrode bulk and electrode/electrolyte interface. In this work, an avenue is opened for designing III-nitrides/Cu2O heterojunction toward solar energy conversion.