We report high-performance ZnO thin-film transistor (ZnO-TFT) fabricated by rf magnetron sputtering at room temperature with a bottom gate configuration. The ZnO-TFT operates in the enhancement mode with a threshold voltage of $19\mathrm{V}$ , a saturation mobility of $27{\mathrm{cm}}^2∕\mathrm{V}\mathrm{s}$ , a gate voltage swing of $1.39\mathrm{V}∕\text{decade}$ and an on/off ratio of $3\times {10}^5$ . The ZnO-TFT presents an average optical transmission (including the glass substrate) of 80% in the visible part of the spectrum. The combination of transparency, high mobility, and room-temperature processing makes the ZnO-TFT a very promising low-cost optoelectronic device for the next generation of invisible and flexible electronics.