AbstractHigh‐throughput (HT) synthesis and HT characterization techniques are becoming increasingly important due to the ever‐increasing complexity of materials and applications of advanced functional compounds. This work reports on the high‐throughput compilation of material libraries of high‐entropy oxides with fluorite crystal structure and tunable band gaps to be used as, e.g., semiconductors for photovoltaic applications. The material libraries cover the high‐entropy range of rare‐earth oxides with 5, 6, and 7 different cations (Ce, La, Sm, Pr, Tb, Y, and Zr) in near equimolar concentrations, but also the medium entropy range with 4 cations. The atmosphere used during or after synthesis is found to have a large effect on the band gap of these materials. Multivalent rare‐earth cations such as Ce/Pr/Tb enable reversible tuning of the band gap between 2.0 and 3.5 eV upon calcination under various oxidizing and reducing atmospheres. The high‐entropy fluorite oxides with smaller band gaps exhibit high electron mobility and transport energy levels compatible with common solar cell architectures.