Sesquioxides such as Y₂O₃ and Sc₂O₃ are important optical materials, but the fabrication of their transparent ceramics remains a challenge due to the ultra-high melting point of over 2400 ℃. In this work, a series of (Y_1-xSc_x)₂O₃ transparent ceramics were successfully fabricated by a simple vacuum sintering process without any sintering additives, and the effect of scandium (Sc) content (x) on the crystal structure and optical/thermal/mechanical properties was evaluated. Y₂O₃ and Sc₂O₃ form a complete solid solution with a cubic bixbyite structure. The formation of (Y_1-xSc_x)₂O₃ solid solution promotes the densification of ceramics, leading to the realization of high transparency close to the theoretical transmittance over a wide wavelength range of 0.35-8 µm. In particular, the in-line transmittance in the range of 0.6-6 µm remains above 80% for (Y_1-xSc_x)₂O₃ with x = 0.23-0.31, while the pristine Y₂O₃ and Sc₂O₃ are opaque. Moreover, the mechanical properties including Vickers hardness (HV), fracture toughness (K_IC), and biaxial flexural strength (δ_b) are evidently enhanced due to the solid solution strengthening, while the thermal conductivity (k) is reduced due to the reduction of photon free path. This study demonstrates that forming of solid solution is a facile and universal approach for preparing sesquioxide transparent ceramics with high optical and mechanical quality.