In this work, the CuI thin film is introduced as an alternative back contact layer (BCL) to the expensive Au BCL for the Cu 2 O photocathodes. It is fabricated by a solution process using CuI precursor solution, dissolving CuI powders in acetonitrile. The thickness of CuI BCL is optimized by controlling the concentration of CuI precursor solution. As a result, the Cu 2 O photocathodes based on the CuI BCL with the optimal thickness (approximately 6.4 nm) show a comparable photoelectrochemical performance compared to the Cu 2 O photocathodes based on the traditional Au BCL. The solution‐processed CuI thin film properly works as a BCL for the Cu 2 O photocathodes by allowing the effective hole transport from Cu 2 O to the back contact and suppressing the hole–electron recombination at the back contact via the large conduction band offset at the CuI/Cu 2 O interface. This provides a novel and promising approach to develop the photocathode with entirely low‐cost materials for solar water splitting.