p-Type delafossite CuFeO2 is a promising material for photoelectrochemical water reduction due to its significant light absorption, chemical stability, and earth abundance. Herein, the CuFeO2 nanoflakes are synthesized by using a sodium alginate-assisted hydrothermal method in a strong alkali environment with NaOH as the mineralizer. The carboxyl and hydroxyl group of sodium alginate adsorbs on the exposed facets of CuFeO2 and promotes the generation of nanoflakes with small thickness. Moreover, the thin CuFeO2 nanoflakes provide a direct and short charge transport channel for photoinduced charge carriers with an improved photoelectrochemical water reduction performance.