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Abstract As one of the most efficient photocathodes, Cu2O has attracted substantial attention because of its theoretically high solar‐to‐hydrogen (STH) efficiency. However, its applications in photoelectrochemical (PEC) fields are severely restricted by the poor stability derived from serious photocorrosion. In this work, high‐quality phenylethynyl copper (PhCCCu) are successfully self‐assembled on the surface of Cu2O photocathode by a novel photothermal method to improve its photostability. With the protection of the PhCCCu layer, 85% of the initial photocurrent density can be remained, while only 28% of initial photocurrent density is left on bare Cu2O photocathode prepared on a copper foam (CF) substrate. The significantly improved photostability of Cu2O photocathode by the PhCCCu protective layer is attributed to its strong hydrophobicity, which can efficiently inhibit the corrosion of Cu2O by the aqueous electrolyte solution due to its special crystal structure. Based on the obtained PhCCCu/Cu2O photocathode, a two‐photoelectrode cell with excellent stability (>5 h) has also been successfully constructed for water splitting without the need of an electric bias.