Binary and ternary II–VI group semiconductor compounds are materials with potential use in various optoelectronic applications, including photoelectrochemical (PEC) solar cells. Thin films of ZnS0.2Se0.8 with various Fe-doping concentrations were successfully deposited on fluorine-doped tin oxide-coated glass substrates at a deposition temperature of 275 °C using the chemical spray pyrolysis. PEC cells with Fe:ZnS0.2Se0.8 thin film/1 M polysulphide/C (graphite) configurations were designed, and the effect of Fe doping on the PEC performance was studied. The results showed that Fe doping in ZnS0.2Se0.8 enhanced the performance of the PEC cells significantly. The optimum concentration was 0.20 mol%. The flat band potential and junction barrier height were maximum at this concentration, with values of −1.18 V and 0.27 eV, respectively. The junction ideality factors of the 0.20 mol% Fe-doped ZnS0.2Se0.8 thin film-based PEC cell in the dark and under illumination were found to be 1.21 and 1.17, respectively. The photovoltaic power output characteristics were boosted by Fe doping with a concentration of 0.20 mol%, with the open circuit voltage being 320 mV and the short circuit current being 1.48 mA cm−2. The solar-to-electrical conversion and the fill factor of the 0.20 mol% Fe-doped ZnS0.2Se0.8 thin film-based PEC cells were superior, with values of 2.84% and 0.60, respectively. A PEC cell with a 0.20 mol% Fe-doped ZnS0.2Se0.8 photosensitive thin film had the highest spectral sensitivity, at a wavelength of 375 nm, with an optical band gap of 3.30 eV.