Abstract In this study, a high-entropy approach was used to design a highly transparent (81.3% in-line transmittance) sesquioxide X2O3 ceramics, in which a single-phase solid solution, with the general formula (Lu0.2Y0.395Gd0.2Yb0.2Tm0.005)2O3, was obtained after sintering under vacuum with La2O3 and ZrO2 as the sintering additives. Upconversion emission from Tm3+ ion at 485 nm was demonstrated upon the 980 nm laser emission excitation as a result of cooperative energy transfer from the Yb3+ ions. High-resolution scanning electron microscope (SEM) and Energy Dispersive X-Ray Spectroscopy (EDS) elemental mapping showed that the obtained transparent ceramics were pore-free and without segregation of different elements.