We studied high-k soluble yttrium oxide dielectrics for high performance oxide thin film transistors (TFTs). The electrical characteristics of yttrium oxide show leakage current density as less than 10⁻⁶ A cm⁻² at 2 MV cm⁻¹ regardless of annealing temperature and a high dielectric constant of nearly 16. For the first time, all solution-processed fully transparent ZnO-TFTs based on spin-coated YO_x gate dielectric layers with a small interfacial trap density and high capacitance were demonstrated, exhibiting a field-effect mobility of 135 cm² V⁻¹ s⁻¹, a threshold voltage of 1.73 V, and an on–off current ratio of 5.7 × 10⁷ as well as low-voltage operation. In addition to microstructural and electrical analyses for solution-processed YO_x dielectrics, we investigated the influences of dielectric–semiconductor interfacial quality on device parameters. Our results suggest that solution-processable fully transparent oxide TFTs have the potential for low-temperature and high-performance application in transparent, flexible devices.