Red-light-emitting phosphors capable of being well excited with blue light are highly desirable in solid-state lighting. In this work, a novel Eu³⁺-activated high-entropy rare earth oxide Ce_0.2La_0.2Gd_0.2Y_0.2Lu_0.2O_1.6:xEu³⁺ (x = 4–16 mol%) phosphor was successfully prepared by solution combustion reaction for the first time. The multi-composition rare earth oxide has a specific cubic fluorite structure, which is almost the same as that of the pure CeO₂ despite the tiny ceria composition in the sample, demonstrating the formation of a high-entropy composite solid solution. To our surprise, the high-entropy phosphor exhibits extremely intense red emission at 613 nm, corresponding to the ⁵D₀→⁷F₂ characteristic transition of Eu³⁺ under the excitation of blue light at 466 nm. The luminescence internal quantum yield (QY) for the optimal high-entropy phosphor (x = 12 mol%) reaches nearly 50% and can further increase to 67.8% through a subsequent heat-treatment process at 1400 °C. The QY result is much superior to that of previously reported Eu³⁺-activated CeO₂ as well as Y₂Ce₂O₇ and La₂Ce₂O₇ low-entropy composite oxides (QYs are approximately 10%–20%). Moreover, the high-entropy oxide phosphor also shows better luminescence thermal stability than low-entropy oxides, as confirmed from the temperature-dependent photoluminescence emission spectra. The tremendous improvement in optical properties depends closely upon the high-entropy and other related effects. The novel high-entropy rare earth oxide phosphor is beneficial to be used in the field of solid-state lighting owing to the coincidence of excitation of blue light with the emission of InGaN light-emitting diode (LED) chips.